AMDGPU: Support GDS atomics

Summary:
Original patch by Marek Olšák

Change-Id: Ia97d5d685a63a377d86e82942436d1fe6e429bab

Reviewers: mareko, arsenm, rampitec

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D63452

llvm-svn: 364814