[llvm] [SelectionDAG] Optimize MPI for align(1) GEPs using base pointer (PR #145309)

[Acthinks Yang via llvm-commits]

Acthinks wrote:

Thanks to @nikic  @arsenm for the suggestion. If this method is feasible, I will continue to optimize the TODO items.

https://github.com/llvm/llvm-project/pull/145309