[AMDGPU] SDWA peephole: enable by default

Reviewers: vpykhtin, rampitec, arsenm

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

Differential Revision: https://reviews.llvm.org/D31671

llvm-svn: 299536