[AMDGPU] Mark ds instructions as meybeAtomic

These were not recognized as potential atomics by memory legalizer.
The test was working not because legalizer did a right thing, but
because it has skipped all these instructions. When I have fixed
DS desciption test started to fail because region address has
changed from 4 to 2 a while ago.

Differential Revision: https://reviews.llvm.org/D58802

llvm-svn: 355179