[AMDGPU] SDWA: make pass global

Summary: Remove checks for basic blocks.

Reviewers: vpykhtin, rampitec, arsenm

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

Differential Revision: https://reviews.llvm.org/D31935

llvm-svn: 300040