Adding x64 simd assembler support.

src/x64/assembler-x64-inl.h

Index: src/x64/assembler-x64-inl.h
diff --git a/src/x64/assembler-x64-inl.h b/src/x64/assembler-x64-inl.h
index 0af8f9393642f39c08d1d339990ceefa506bffc9..4fc9f8b544fa7853d8f7629a393aef1cfcbe5bf5 100644
--- a/src/x64/assembler-x64-inl.h
+++ b/src/x64/assembler-x64-inl.h
@@ -179,12 +179,11 @@ void Assembler::emit_optional_rex_32(Register rm_reg) {
   if (rm_reg.high_bit()) emit(0x41);
 }
 
-
-void Assembler::emit_optional_rex_32(XMMRegister rm_reg) {
-  if (rm_reg.high_bit()) emit(0x41);
+void Assembler::emit_optional_rex_32(XMMRegister reg) {
+  byte rex_bits = (reg.code() & 0x8) >> 1;
+  if (rex_bits != 0) emit(0x40 | rex_bits);
 }
 
-
 void Assembler::emit_optional_rex_32(const Operand& op) {
   if (op.rex_ != 0) emit(0x40 | op.rex_);
 }