[AMDGPU] Simplify test and extend to gfx9 and gfx10

Summary:
This is in preparation for adding more test cases for D69661 and other
bug fixes in the same area.

Reviewers: tpr, dstuttard, critson, nhaehnle, arsenm

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D70708