[AMDGPU] Assembler: SDWA/DPP should not accept scalar registers and immediate operands

Reviewers: artem.tamazov, nhaustov, vpykhtin, tstellarAMD

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

Differential Revision: https://reviews.llvm.org/D28157

llvm-svn: 291668