[x86 asm] allow fwait alias in both At&t and Intel modes (PR21208)

Differential Revision: http://reviews.llvm.org/D5741
[FastISel][AArch64] Add custom lowering for GEPs.

This is mostly a copy of the existing FastISel GEP code, but on AArch64 we bail
out even for simple cases, because the standard fastEmit functions don't cover
MUL and ADD is lowered inefficientily.