[AMDGPU] SDWA: add disassembler support for GFX9

Summary: Added decoder methods and tests

Reviewers: vpykhtin, artem.tamazov, dp

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

Differential Revision: https://reviews.llvm.org/D33545

llvm-svn: 303999