AMDGPU: Consolidate SMRD TableGen patterns

Summary:
Merge the SMRD patterns for CI into the same multiclass as the
patterns for other sub-targets.

This removes some duplicate code and will make it easier for some
future GlobalISel changes I would like to do.

Reviewers: arsenm

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

Differential Revision: https://reviews.llvm.org/D52557

llvm-svn: 343909