Encode MOV $0, %ax as XOR %eax, %eax instead of
XOR %rax, %rax. If an operand register does not
need REX.w bit (i.e. not one of R8-R15), it is
encoded in 2 bytes instead of 3 bytes.

LGTM=rsc
R=golang-codereviews, gobot, rsc
CC=golang-codereviews
https://golang.org/cl/115580044