[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

llvm/lib/Target/AMDGPU/DSInstructions.td

@@ -18,6 +18,7 @@ class DS_Pseudo <string opName, dag outs, dag ins, string asmOps, list<dag> patt
   // Most instruction load and store data, so set this as the default.
   let mayLoad = 1;
   let mayStore = 1;
+  let maybeAtomic = 1;
 
   let hasSideEffects = 0;
   let SchedRW = [WriteLDS];