[llvm] [SROA] Improve select folding around casts (PR #222466)

[Daniel Donenfeld via llvm-commits]

daniel-donenfeld wrote:

@yxsamliu I updated the branch to look through GEPs, addr and bit casts, or selects when searching for volatile instructions. I added it as a helper, as I have a similar change to unfoldGEPPhi (https://github.com/llvm/llvm-project/pull/227047) that I believe needs the same logic

https://github.com/llvm/llvm-project/pull/222466