X64: Many small tweaks and more usages of "load smi as int32".

src/x64/macro-assembler-x64.h

Index: src/x64/macro-assembler-x64.h
diff --git a/src/x64/macro-assembler-x64.h b/src/x64/macro-assembler-x64.h
index b4f3240ec819e115a8b70738abcebc99e05fd085..2340079ddde6228780bf64c75fceb372ef5d8bbc 100644
--- a/src/x64/macro-assembler-x64.h
+++ b/src/x64/macro-assembler-x64.h
@@ -210,6 +210,7 @@ class MacroAssembler: public Assembler {
   // Convert smi to 32-bit integer. I.e., not sign extended into
   // high 32 bits of destination.
   void SmiToInteger32(Register dst, Register src);
+  void SmiToInteger32(Register dst, const Operand& src);
 
   // Convert smi to 64-bit integer (sign extended if necessary).
   void SmiToInteger64(Register dst, Register src);