Fix simulation of fcvtn

The logic for fcvtn was failing to clear the top half of the vector register, so
add the missing ClearForWrite(), plus a regression test.

Reported by Yury Delendik, from https://bugzilla.mozilla.org/show_bug.cgi?id=1697846
diff --git a/test/test-trace-reference/log-state b/test/test-trace-reference/log-state
index 8113a01..b495bfe 100644
--- a/test/test-trace-reference/log-state
+++ b/test/test-trace-reference/log-state
@@ -5088,7 +5088,7 @@
 #            v26: 0x00000000000000000000000000000000
 #            v21: 0x00000000000000000000000000000000
 #            v11: 0x00000000000000000000000000000000
-#             v8: 0x37a00000000000000000000000000000
+#             v8: 0x00000000000000000000000000000000
 #            v24: 0x0000000000000000377f0000377f0000
 #             v4: 0x0000000000000000ffffffffffffffff
 #            v25: 0x00000000000000000000000000000000
@@ -5117,24 +5117,24 @@
 #             v9: 0x00000000000000000000000000000000
 #            v30: 0x00000000000000000000000000000000
 #            v19: 0x00000000000000000000000000000000
-#            v15: 0x7ff00000000000007ff8000000000000
+#            v15: 0x7ff80000000000007ff8000000000000
 #            v12: 0x00000000000000007fc000007fc00000
 #            v19: 0xffffffffffffffffffffffffffffffff
-#            v19: 0x37a00000000000000000000000000000
+#            v19: 0x00000000000000000000000000000000
 #            v25: 0x00000000000000007fc000007fc00000
-#             v6: 0x7ff00000000000007ff8000000000000
-#            v16: 0x37a0000000000000000000000180fe00
+#             v6: 0x7ff80000000000007ff8000000000000
+#            v16: 0x0000000000000000000000000180fe00
 #            v15: 0x00000000000000000000000000000000
-#            v23: 0x37a0000000000000000000000180fe00
-#             v6: 0x000000000000000037a0000000000000
+#            v23: 0x0000000000000000000000000180fe00
+#             v6: 0x00000000000000000000000000000000
 #            v27: 0x00000000000000000000000000000000
-#             v8: 0x37a00000000000007fc000007fc00000
+#             v8: 0x000000000180fe007fc000007fc00000
 #            v13: 0x0000000000000000ffffffff7fc00000
 #            v15: 0xffffffff000000000000000000000000
-#            v27: 0x00000000000000000000000037a00000
+#            v27: 0x00000000000000000000000000000000
 #            v20: 0x0000000000000000ffffffff00000000
 #            v18: 0x00000000000000000000000000000000
-#             v9: 0x00000000ffffffff37a0000000000000
+#             v9: 0x00000000ffffffff000000000180fe00
 #             v7: 0x0000000000000000ffffffffffffffff
 #            v18: 0x000000000000000000000000ffffffff
 #            v31: 0x00000000000000000000000000000000
@@ -5163,14 +5163,14 @@
 #            v19: 0x00000000000000007fc000007fc00000
 #            v24: 0x00000000000000000000000000000000
 #            v16: 0x00000000000000000000000000000000
-#            v27: 0x00000000000000000000000037a00000
-#            v27: 0x00000000000000000000000037a00000
+#            v27: 0x00000000000000000000000000000000
+#            v27: 0x00000000000000000000000000000000
 #            v21: 0x00000000000000000000000000000000
 #             v5: 0x00000000000000000000000000000000
 #            v18: 0x000000000000000000000000ffffffff
 #             v5: 0x00000000000000000000000000000000
 #             v3: 0x0000000000000000000000007fffffff
-#            v27: 0x00000000000000000000000037a00000
+#            v27: 0x00000000000000000000000000000000
 #            v26: 0x00000000000000007fffffff00000000
 #            v14: 0xbfd6000000000000bfd6000000000000
 #            v26: 0x00000000000000003f6800003f680000
@@ -5198,7 +5198,7 @@
 #             v5: 0x80000000800000008000000080000000
 #            v18: 0x7ff00000000000007ff0000000000000
 #            v10: 0x00000000000000007f8000007f800000
-#             v5: 0x7f8000007f800000474c80007f800000
+#             v5: 0x7f8000007f8000007f8000007f800000
 #            v22: 0x40000000000000004000000000000000
 #            v31: 0x00000000000000004000000040000000
 #            v18: 0x40000000400000004000000040000000
@@ -5795,9 +5795,9 @@
 #   z24<639:512>: 0x00000000000000000000000000000000 (0.000, 0.000)
 #                                  ║               ╙─ 0x0000000000000000'0000000000000000 <- 0x~~~~~~~~~~~~~~~~
 #                                  ╙───────────────── 0x0000000000000000'0000000000000000 <- 0x~~~~~~~~~~~~~~~~
-#     z24<127:0>: 0x0000000000000000000000ff00000000
-#     z25<127:0>: 0xa000000000000000000000ffc0000000
-#     z26<127:0>: 0x3700000000000000000000007f000000
+#     z24<127:0>: 0x00000000fe000000000000ff00000000
+#     z25<127:0>: 0x0000000080000000000000ffc0000000
+#     z26<127:0>: 0x0000000001000000000000007f000000
 #                    ║ ║   ║ ║ ║ ║ ║ ║   ║ ║ ║ ║ ║ ╙─ 0x00'00'00 <- 0x~~~~~~~~~~~~~~~~
 #                    ║ ║   ║ ║ ║ ║ ║ ║   ║ ║ ║ ║ ╙─── 0x00'00'00 <- 0x~~~~~~~~~~~~~~~~
 #                    ║ ║   ║ ║ ║ ║ ║ ║   ║ ║ ║ ╙───── 0x00'00'00 <- 0x~~~~~~~~~~~~~~~~
@@ -5808,10 +5808,10 @@
 #                    ║ ║   ║ ║ ║ ║ ╙───────────────── 0x00'00'00 <- 0x~~~~~~~~~~~~~~~~
 #                    ║ ║   ║ ║ ║ ╙─────────────────── 0x00'00'00 <- 0x~~~~~~~~~~~~~~~~
 #                    ║ ║   ║ ║ ╙───────────────────── 0x00'00'00 <- 0x~~~~~~~~~~~~~~~~
-#                    ║ ║   ║ ╙─────────────────────── 0x00'00'00 <- 0x~~~~~~~~~~~~~~~~
+#                    ║ ║   ║ ╙─────────────────────── 0x01'80'fe <- 0x~~~~~~~~~~~~~~~~
 #                    ║ ║   ╙───────────────────────── 0x00'00'00 <- 0x~~~~~~~~~~~~~~~~
 #                    ║ ╙───────────────────────────── 0x00'00'00 <- 0x~~~~~~~~~~~~~~~~
-#                    ╙─────────────────────────────── 0x37'a0'00 <- 0x~~~~~~~~~~~~~~~~
+#                    ╙─────────────────────────────── 0x00'00'00 <- 0x~~~~~~~~~~~~~~~~
 #   z24<255:128>: 0x00000000000000000000000000000000
 #   z25<255:128>: 0x00000000000000000000000000000000
 #   z26<255:128>: 0x00000000000000000000000000000000
@@ -5936,11 +5936,11 @@
 #                      ╙───────────────────────────── 0xedec'ebea'e9e8 <- 0x~~~~~~~~~~~~~~~~
 #     z26<127:0>: 0x00000000000000000000ffff00000000 (0.000, 0.000, 9.183e-41, 0.000)
 #     z27<127:0>: 0x00000000000000007fff000000000000 (0.000, 0.000, 9.183e-41, 0.000)
-#     z28<127:0>: 0x37a0000000000000000000007fc00000 (0.000, 0.000, 9.183e-41, 0.000)
+#     z28<127:0>: 0x000000000180fe00000000007fc00000 (0.000, 0.000, 9.183e-41, 0.000)
 #                          ║       ║       ║       ╙─ 0x7fc00000'00000000'00000000 <- 0x~~~~~~~~~~~~~~~~
 #                          ║       ║       ╙───────── 0x00000000'7fff0000'0000ffff <- 0x~~~~~~~~~~~~~~~~
-#                          ║       ╙───────────────── 0x00000000'00000000'00000000 <- 0x~~~~~~~~~~~~~~~~
-#                          ╙───────────────────────── 0x37a00000'00000000'00000000 <- 0x~~~~~~~~~~~~~~~~
+#                          ║       ╙───────────────── 0x0180fe00'00000000'00000000 <- 0x~~~~~~~~~~~~~~~~
+#                          ╙───────────────────────── 0x00000000'00000000'00000000 <- 0x~~~~~~~~~~~~~~~~
 #   z26<255:128>: 0x00000000000000000000000000000000 (0.000, 0.000, 0.000, 0.000)
 #   z27<255:128>: 0x00000000000000000000000000000000 (0.000, 0.000, 0.000, 0.000)
 #   z28<255:128>: 0x00000000000000000000000000000000 (0.000, 0.000, 0.000, 0.000)
@@ -5970,10 +5970,10 @@
 #                          ║       ╙───────────────── 0x00000000'00000000'00000000 <- 0x~~~~~~~~~~~~~~~~
 #                          ╙───────────────────────── 0x00000000'00000000'00000000 <- 0x~~~~~~~~~~~~~~~~
 #     z27<127:0>: 0x00000000000000000000000000000000 (0.000, 0.000)
-#     z28<127:0>: 0x00000000000000000000ffff7fc00000 (0.000, 0.000)
-#     z29<127:0>: 0x37a0000000000000000000007fff0000 (0.000, 0.000)
+#     z28<127:0>: 0x000000000180fe000000ffff7fc00000 (0.000, 0.000)
+#     z29<127:0>: 0x0000000000000000000000007fff0000 (0.000, 0.000)
 #                                  ║               ╙─ 0x000000007fff0000'0000ffff7fc00000'0000000000000000 <- 0x~~~~~~~~~~~~~~~~
-#                                  ╙───────────────── 0x37a0000000000000'0000000000000000'0000000000000000 <- 0x~~~~~~~~~~~~~~~~
+#                                  ╙───────────────── 0x0000000000000000'000000000180fe00'0000000000000000 <- 0x~~~~~~~~~~~~~~~~
 #   z27<255:128>: 0x00000000000000000000000000000000 (0.000, 0.000)
 #   z28<255:128>: 0x00000000000000000000000000000000 (0.000, 0.000)
 #   z29<255:128>: 0x00000000000000000000000000000000 (0.000, 0.000)