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

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.


Committed: http://code.google.com/p/v8/source/detail?r=3132

[fschneider, 2009-10-26 12:28:57]

This is a small optimization for x64. 

It saves 1 byte in encoding xor reg, reg (i.e. when using xor to zero a
register).

[William Hesse, 2009-10-26 13:44:40]

LGTM.