AMDGPU/GlobalISel: Add support for simple shaders

Summary: We can select constant/global G_LOAD, global G_STORE, and G_GEP.

Reviewers: qcolombet, MatzeB, t.p.northover, ab, arsenm

Subscribers: mehdi_amini, vkalintiris, kzhuravl, wdng, nhaehnle, mgorny, yaxunl, tony-tye, modocache, llvm-commits, dberris

Differential Revision: https://reviews.llvm.org/D26730

llvm-svn: 293503