Fixes (keep it open) #130110.
If the incoming value is PHI itself, we can skip this. If we can
guarantee that the other incoming values are neither undef nor poison,
then we can also guarantee that the value isn't either. If we cannot
guarantee that, it makes no sense in calculating it.
(cherry picked from commit 462eb7e28ef4507b16a4b45efb356bc6a3523615)