[Tests] Add tests for all idemptotent atomicrmws

Base for a followup patch to strengthen the InstCombine transform, and then integrate the ExpandAtomics logic.



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@354036 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed