AMDGPU: Don't look for constant in insert/extract_vector_elt regbankselect

The constantness shouldn't change the register bank choice. We also
don't need to restrict this to only indexing VGPRs, since it's
possible to index SGPRs (but SelectionDAG made using this
difficult). Allow directly indexing SGPRs when appropriate.

llvm-svn: 356611