[AMDGPU] Optimize atomic max/min

Summary:
Extend the atomic optimizer to handle signed and unsigned max and min
operations, as well as add and subtract.

Reviewers: arsenm, sheredom, critson, rampitec

Subscribers: kzhuravl, jvesely, wdng, nhaehnle, yaxunl, dstuttard, tpr, t-tye, hiraditya, jfb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64328

llvm-svn: 366235