AH is a thorn in the flesh for our X86-64 backend. The assembler was
designed to always encode the low 8-bit registers, so %ah would become
%spl. While it is true we **could** force %spl to always be encoded as
%ah, that would not work if the instruction has a rex prefix.

This CL removes references to %ah from TargetX86Base. There used to be
2 uses of ah in the target lowering:

1) To zero-extend %al before an unsigned div:
    mov <<src0>>, %al
    mov 0, %ah
    div <<src1>>

This pattern has been changed to
    xor %eax, %eax
    mov <<src0>>, %al
    div <<src1>>

2) To access the 8-bit remainder for 8-bit division:
    mov %ah, <<dest>>

This pattern has been changed to
    shr $8, %eax
    mov %al, <<Dest>>

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4077
R=stichnot@chromium.org

Review URL: https://codereview.chromium.org/1260163003.