AMDGPU: Improve alias analysis for GDS

Summary: GDS cannot alias anything else.

Original patch by: Marek Olšák

Reviewers: arsenm, mareko

Subscribers: kzhuravl, jvesely, wdng, yaxunl, dstuttard, tpr, t-tye, Petar.Avramovic, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64114

Change-Id: I07bfbd96f5d5c37a6dfba7997df12f291dd794b0
llvm-svn: 366313