[llvm] [InstCombine] Fold `X!=Y ? ctz(X^Y, true) : BW -> ctz(X^Y, false)` (PR #128483)

[Nikita Popov via llvm-commits]

nikic wrote:

> > In terms of high-level approach, an alternative I'd like to consider is to instead add support for the ctz/abs poison flag to hasPoisonGeneratingAnnotations / dropPoisonGeneratingAnnotations. I believe that way we'll automatically handle this through the select equivalence fold.
> > (I think that may allow removing this special case fold in InstCombine entirely?)
> 
> Do you mean we can perform this fold in `simplifyWithOpReplaced`?

Yes. We have a variant of simplifyWithOpReplaced that ignores poison annotations and then drops them if the fold would succeed. I believe it would handle this case.

https://github.com/llvm/llvm-project/pull/128483