[llvm] [AMDGPU] Fold multiple aligned v_mov_b32 to v_mov_b64 on gfx942 (PR #138843)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

We probably should be selecting these cases directly to the pseudo up-front. It's easier to fold subregister uses of the wider def than this 

https://github.com/llvm/llvm-project/pull/138843