[llvm] [ConstraintElim] Fix poison check before adding intrinsic facts (PR #136291)

[Nikita Popov via llvm-commits]

nikic wrote:

> > Extending this check to abs is problematic due to the interaction with the int_min_is_poison flag.
> 
> Then there's no functional change in this patch.

Right. I think it's okay to add the poison check workaround for usub_sat to unblock the patch stack, but I don't think we should touch abs until we can handle this properly.

https://github.com/llvm/llvm-project/pull/136291