[AMDGPU] Add pseudo SDWA instructions

Summary: This is needed for later SDWA support in CodeGen.

Reviewers: vpykhtin, tstellarAMD

Subscribers: arsenm, kzhuravl, wdng, nhaehnle, yaxunl, tony-tye

Differential Revision: https://reviews.llvm.org/D27412

llvm-svn: 290338