[llvm] [AMDGPU] Add `isAsCheapAsAMove` for `v_pk_mov_b32` (PR #127632)

[Matt Arsenault via llvm-commits]

arsenm wrote:

Best bet is likely a rematerialize test. e.g. add a case to remat-vop.mir that needs to handle v_pk_mov_b32 

https://github.com/llvm/llvm-project/pull/127632