Misc optimizer+codegen work for 'cmpxchg' and 'atomicrmw'. They appear to be

Misc optimizer+codegen work for 'cmpxchg' and 'atomicrmw'.  They appear to be
working on x86 (at least for trivial testcases); other architectures will
need more work so that they actually emit the appropriate instructions for
orderings stricter than 'monotonic'. (As far as I can tell, the ARM, PPC,
Mips, and Alpha backends need such changes.)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@136457 91177308-0d34-0410-b5e6-96231b3b80d8