AMDGPU/GlobalISel: Move SMRD selection logic to TableGen

Reviewers: arsenm

Reviewed By: arsenm

Subscribers: volkan, kzhuravl, jvesely, wdng, nhaehnle, yaxunl, rovka, kristof.beyls, dstuttard, tpr, t-tye, llvm-commits

Differential Revision: https://reviews.llvm.org/D52922

llvm-svn: 354516