Do not assume rtti by using dynamic_cast.

We already have the MacroAssemblerInterface* we need in
EmissionCheckScope.

Change-Id: I10b74594a73850f33eea50a40804abc88665f40b
diff --git a/src/code-generation-scopes-vixl.h b/src/code-generation-scopes-vixl.h
index eae5bde..f880b6c 100644
--- a/src/code-generation-scopes-vixl.h
+++ b/src/code-generation-scopes-vixl.h
@@ -256,9 +256,7 @@
 
   virtual ~ExactAssemblyScope() {
 #ifdef VIXL_DEBUG
-    MacroAssemblerInterface* masm =
-        dynamic_cast<MacroAssemblerInterface*>(assembler_);
-    masm->SetAllowMacroInstructions(previous_allow_macro_assembler_);
+    masm_->SetAllowMacroInstructions(previous_allow_macro_assembler_);
 #endif
   }