[NFCI][SCEV] getPtrToIntExpr(): use SCEVRewriteVisitor<> for ptrtoint cast sinking

This is functionally-identical to the previous implementation,
just using a generic interface to do that instead of hand-rolled one,
with caching as a bonus. Thought the sinking is still recursive..

Note that SCEVRewriteVisitor<>'s default implementations
don't preserve NoWrap flags on Add/Mul (but does on AddRec!),
but here we know we can preserve them,
so `visitAddExpr()`/`visitMulExpr()` are specialized.