AMDGPU: Add/Fix tests for image atomic intrinsic.

Summary:
Add tests for 64-bit image atomic swap and cmpswap.
Fix tests for 32-bit image atomic add.

Change-Id: Ibb7619749c1ad504b24aa1c5f3185417a3013f3c

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D75295