[InstCombine] Simplify selects that test cmpxchg instructions

If a select instruction tests the returned flag of a cmpxchg instruction and
selects between the returned value of the cmpxchg instruction and its compare
operand, the result of the select will always be equal to its false value.

Differential Revision: https://reviews.llvm.org/D39383

llvm-svn: 316994