[InstCombine] Optimize `atomicrmw <op>, 0` into `load atomic` when possible

[InstCombine] Optimize `atomicrmw <op>, 0` into `load atomic` when possible

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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353471 91177308-0d34-0410-b5e6-96231b3b80d8