[ConstraintElim] Make sure min/max intrinsic results are not poison.

The result of umin may be poison and in that case the added constraints
are not be valid in contexts where poison doesn't cause UB. Only queue
facts for min/max intrinsics if the result is guaranteed to not be
poison.

This could be improved in the future, by only adding the fact when
solving conditions using the result value.

Fixes https://github.com/llvm/llvm-project/issues/78621.