[Tests] RMW folding tests w/unordered atomic operations

We get a suprising number of these today actually, but some are missed. The main point of this is strengthen the test set for D57601.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353966 91177308-0d34-0410-b5e6-96231b3b80d8