[AMDGPU] stop image_store being moved illegally

Summary:
A recent change
321556: AMDGPU: Remove mayLoad/hasSideEffects from MIMG stores
can allow the machine instruction scheduler to move an image store past
an image load using the same descriptor.

V2: Fixed by marking image ops as mayAlias and isAliased. This may be
overly conservative, and we may need to revisit.
V3: Reverted test change done on 321556.

Reviewers: arsenm, nhaehnle, dstuttard

Subscribers: llvm-commits, t-tye, yaxunl, wdng, kzhuravl

Differential Revision: https://reviews.llvm.org/D41969

llvm-svn: 322419