AMDGPU: add missing llvm.amdgcn.{raw,struct}.buffer.atomic.{inc,dec}

Summary:
Wrapping increment/decrement. These aren't exposed by many APIs...

Change-Id: I1df25c7889de5a5ba76468ad8e8a2597efa9af6c

Reviewers: arsenm, tpr, dstuttard

Subscribers: kzhuravl, jvesely, wdng, yaxunl, t-tye, jfb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65283

llvm-svn: 367821