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

src/x64/assembler-x64.h

 // Copyright (c) 1994-2006 Sun Microsystems Inc.
 // All Rights Reserved.
 //
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 // - Redistributions of source code must retain the above copyright notice,
 // this list of conditions and the following disclaimer.
 //
@@ skipping 113 matching lines @@
     };
     return names[index];
   }
 
   static Register from_code(int code) {
     Register r = { code };
     return r;
   }
   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_;
   }
   int bit() const {
     return 1 << code_;
   }
 
   // Return the high bit of the register code as a 0 or 1.  Used often
   // when constructing the REX prefix byte.
@@ skipping 1500 matching lines @@
  private:
   Assembler* assembler_;
 #ifdef DEBUG
   int space_before_;
 #endif
 };
 
 } }  // namespace v8::internal
 
 #endif  // V8_X64_ASSEMBLER_X64_H_