[AMDGPU] Save some work when an atomic op has no uses

Summary:
In the atomic optimizer, save doing a bunch of work and generating a
bunch of dead IR in the fairly common case where the result of an
atomic op (i.e. the value that was in memory before the atomic op was
performed) is not used. NFC.

Reviewers: arsenm, dstuttard, tpr

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64981

llvm-svn: 366667