[AMDGPU] Assembler: More tests for SDWA instructions. Fix for SDWA float modifiers.

Summary: Depends on D20625

Reviewers: tstellarAMD, vpykhtin, artem.tamazov

Subscribers: arsenm, kzhuravl

Differential Revision: http://reviews.llvm.org/D20674

llvm-svn: 271662