[AMDGPU] Handle OR as operand of raw load/store

Summary:
Use isBaseWithConstantOffset() which handles OR as an operand
to llvm.amdgcn.raw.buffer.load and llvm.amdgcn.raw.buffer.store.

Change-Id: Ifefb9dc5ded8710d333df07ab1900b230e33539a

Reviewers: nhaehnle, mareko, arsenm

Reviewed By: arsenm

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

Differential Revision: https://reviews.llvm.org/D55999

llvm-svn: 350208