[SCEV] Model ptrtoint(SCEVUnknown) cast not as unknown, but as zext/trunc/self of SCEVUnknown
While we indeed can't treat them as no-ops, i believe we can/should
do better than just modelling them as unknown. inttoptr story
is complicated, but for ptrtoint, it seems straight-forward
to model it just as a zext-or-trunc of unknown.
Reviewed By: mkazantsev
Differential Revision: https://reviews.llvm.org/D88806