Fix register trace involving sp and xzr (#59)

Tracing register updates for ldp instructions that use both xzr and sp would
not print the updated stack address. This was visible when popping from the
stack into xzr, to discard an entry. Fix this and update the trace tests.
diff --git a/test/test-trace-reference/log-write-colour b/test/test-trace-reference/log-write-colour
index 7caf1a4..8a43844 100644
--- a/test/test-trace-reference/log-write-colour
+++ b/test/test-trace-reference/log-write-colour
@@ -44,6 +44,11 @@
 #        x3<7:0>:               0xff -> 0x~~~~~~~~~~~~~~~~
 #       x4<15:0>:             0x0001 -> 0x~~~~~~~~~~~~~~~~
 #       x5<15:0>:             0x8500 -> 0x~~~~~~~~~~~~~~~~
+#      x10<63:0>: 0x0000000000000000 -> 0x~~~~~~~~~~~~~~~~
+#      xzr<63:0>: 0x0000000000000000 -> 0x~~~~~~~~~~~~~~~~
+#      xzr<63:0>: 0x0000000000000000 -> 0x~~~~~~~~~~~~~~~~
+#      xzr<63:0>: 0x0000000000000000 -> 0x~~~~~~~~~~~~~~~~
+#      xzr<63:0>: 0x0000000000000000 -> 0x~~~~~~~~~~~~~~~~
 #     v21<127:0>: 0x000000000000000000000000ffff8007
 #                    ║ ║ ║ ║ ║ ║ ║ ║ ║ ║ ║ ║ ║ ║ ║ ╙─ 0x07 -> 0x~~~~~~~~~~~~~~~~
 #                    ║ ║ ║ ║ ║ ║ ║ ║ ║ ║ ║ ║ ║ ║ ╙─── 0x80 -> 0x~~~~~~~~~~~~~~~~