[AMDGPU] Run unmerge combines post regbankselect

RegBankSelect can insert G_UNMERGE_VALUES in a lot of places which
left us with a lot of unmerge/merge pairs that could be simplified.
These often got in the way of pattern matching and made codegen
worse.

This patch:
  - Makes the necessary changes to the merge/unmerge combines so they can run post RegBankSelect
  - Adds relevant unmerge combines to the list of RegBankSelect combines for AMDGPU
  - Updates some tablegen patterns that were missing explicit cross-regbank copies (V_BFI patterns were causing constant bus violations with this change).

This seems to be mostly beneficial for code quality.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D142192