X86 fast-isel: Avoid explicit AH subreg reference for [SU]Rem.

Explicit references to %AH for an i8 remainder instruction can lead to
references to %AH in a REX prefixed instruction, which causes things to
blow up. Do the same thing in FastISel as we do for DAG isel and instead
shift %AX right by 8 bits and then extract the 8-bit subreg from that
result.

rdar://14203849
http://llvm.org/bugs/show_bug.cgi?id=16105
X86: Add comment.