[AMDGPU] [llvm-mc] [VI] Fix encoding of LDS/GDS instructions.

Patch by: Artem Tamazov

Summary: Tests added.

Reviewers: tstellarAMD, arsenm

Subscribers: vpykhtin, SamWot, #llvm-amdgpu-spb

Projects: #llvm-amdgpu-spb

Differential Revision: http://reviews.llvm.org/D17271

llvm-svn: 261558