[SCEV] BuildConstantFromSCEV(): actually properly handle SExt-of-pointer case

As being pointed out by @efriedma in
https://reviews.llvm.org/rGaaafe350bb65#inline-4883
of course we can't just call ptrtoint in sign-extending case
and be done with it, because it will zero-extend.

I'm not sure what i was thinking there.

This is very much not an NFC, however looking at the user of
BuildConstantFromSCEV() i'm not sure how to actually show that
it results in a different constant expression.