[PATCH] X86: elide comparisons after cmpxchg instructions.

[Tim Northover]

The problem was X86ISD::BRCOND getting a stale chain after the original, unlowered cmpxchg had been replaced. I thought the correct value could be reliably inferred from the "Cmp", but that's not actually true in general so we need to save the chain slightly later in the function.

http://reviews.llvm.org/D4083

Files:
  lib/Target/X86/X86ISelLowering.cpp
  lib/Target/X86/X86ISelLowering.h
  test/CodeGen/X86/cmpxchg-i1.ll
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