[SCEV] SCEVPtrToIntExpr simplifications

If we've got an SCEVPtrToIntExpr(op), where op is not an SCEVUnknown,
we want to sink the SCEVPtrToIntExpr into an operand,
so that the operation is performed on integers,
and eventually we end up with just an `SCEVPtrToIntExpr(SCEVUnknown)`.

Reviewed By: mkazantsev

Differential Revision: https://reviews.llvm.org/D89692