[AMDGPU] SDWA: add support for PRESERVE into SDWA peephole.

Summary:

Reviewers: arsenm, vpykhtin, rampitec

Subscribers: kzhuravl, wdng, nhaehnle, mgorny, yaxunl, dstuttard, tpr, t-tye

Differential Revision: https://reviews.llvm.org/D37817

llvm-svn: 319662