[llvm] [RISCV] Introduce new AND combine to expose additional load narrowing opportunities (PR #170483)

[Alex Bradbury via llvm-commits]

asb wrote:

> Makes sense to me, is there something similar we can also do for stores?

I don't believe so. Unless I'm misunderstanding the transformation you're imagining, we'd go from e.g. writing a 32/64-bit value to a narrower one which means we'd end up with different values in the bytes beyond the narrowed store and would need to prove that doesn't impact anything.

I've pushed updates that I believe address all active review comments. Splitting it out to a helper function means we can make better use of the early return pattern. I've also converted to use `sd_match` but that's in the final commit and can be dropped if you prefer the non-sd_match version for some reason.

https://github.com/llvm/llvm-project/pull/170483