Generate more compact XOR on 64-bit architecture when using xor to zero out registers.

When using xor to zero a 64-bit register, generate 32-bit instruction instead.
(according to Intel 64-bit mode coding guidelines) 

previous code for zeroing RAX:
  xor rax, rax  

==>

new code for zeroing RAX: 
  xor eax, eax

The 32-bit operand form has the same semantics: It also zeroes the upper 
32-bit of rax and its encoding uses 1 byte less.

Review URL: http://codereview.chromium.org/330018

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@3132 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

src/x64/assembler-x64.h

@@ -920,8 +920,12 @@ class Assembler : public Malloced {
   void testq(Register dst, Immediate mask);
 
   void xor_(Register dst, Register src) {
+    if (dst.code() == src.code()) {
+      arithmetic_op_32(0x33, dst, src);
+    } else {
       arithmetic_op(0x33, dst, src);
     }
+  }
 
   void xorl(Register dst, Register src) {
     arithmetic_op_32(0x33, dst, src);