[AMDGPU] SDWA: merge VI and GFX9 pseudo instructions

Summary: Previously there were two separate pseudo instruction for SDWA on VI and on GFX9. Created one pseudo instruction that is union of both of them. Added verifier to check that operands conform either VI or GFX9.

Reviewers: dp, arsenm, vpykhtin

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

Differential Revision: https://reviews.llvm.org/D34026

llvm-svn: 305886