AMDGPU/SI: Simplify moveSMRDToVALU()

Summary:
Replace the switch on instruction opcode with a switch on register size.
This way we don't need to update the switch statement when we add new
SMRD variants.

Reviewers: arsenm

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D11601

llvm-svn: 243652