[AMDGPU] stop buffer_store being moved illegally

Summary:
The machine instruction scheduler was illegally moving a buffer store
past a buffer load with the same descriptor and offset. Fixed by marking
buffer ops as mayAlias and isAliased. This may be overly conservative,
and we may need to revisit.

Subscribers: arsenm, kzhuravl, wdng, nhaehnle, yaxunl, dstuttard, t-tye, llvm-commits

Differential Revision: https://reviews.llvm.org/D43332

Change-Id: Iff3173d9e0653e830474546276ab9d30318b8ef7
llvm-svn: 325567