Ensure using byte registers for byte instructions on ia32 and x64.

src/x64/assembler-x64.h

Index: src/x64/assembler-x64.h
diff --git a/src/x64/assembler-x64.h b/src/x64/assembler-x64.h
index 745850d822e68a003a338f689b5755c5545fb252..8e1dc79847bda94e2524a9bc3bdcee5d8ae41388 100644
--- a/src/x64/assembler-x64.h
+++ b/src/x64/assembler-x64.h
@@ -131,6 +131,8 @@ struct Register {
   }
   bool is_valid() const { return 0 <= code_ && code_ < kNumRegisters; }
   bool is(Register reg) const { return code_ == reg.code_; }
+  // rax, rbx, rcx and rdx are byte registers, the rest are not.
+  bool is_byte_register() const { return code_ <= 3; }
   int code() const {
     ASSERT(is_valid());
     return code_;