InstCombine: Rely on cmpxchg's return code when it's strong

Comparing the result of a cmpxchg instruction can be replaced with an
extractvalue of the cmpxchg success indicator.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221498 91177308-0d34-0410-b5e6-96231b3b80d8