[AMDGPU] Assembler: support SDWA for VOPC instructions

Summary: dst_sel and dst_unused disabled for VOPC as they have no effect on result

Reviewers: artem.tamazov, tstellarAMD, vpykhtin

Subscribers: arsenm, kzhuravl

Differential Revision: http://reviews.llvm.org/D21376

llvm-svn: 274340