[llvm] [InstCombine] Check for undef before transforming lshr (mul (X, 2^N + 1)), N -> add (X, lshr(X, N)) (PR #96324)

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AtariDreams wrote:

> TBH I don't really want to fix partial-undef multi-use miscompiles. We don't have any evidence that they have any read-world relevance.
> 
> We typically do not even fix full-undef multi-use miscompiles in pre-existing transforms (we only don't accept them in new ones).

Honestly, that's valid. I will just close this for now and do some more research.

https://github.com/llvm/llvm-project/pull/96324