This commit teaches InstCombine how to replace an atomicrmw operation
into a simple load atomic.
For a given `atomicrmw <op>`, this is possible when:
1. The ordering of that operation is compatible with a load (i.e.,
anything that doesn't have a release semantic).
2. <op> does not modify the value being stored
Differential Revision: https://reviews.llvm.org/D57854
llvm-svn: 353471