[SCEV] Common code for computing trip count in a fixed type [NFC-ish]

This is a follow on to D147117 and D147355. In both cases, we were adding special cases to compute zext(BTC+1) instead of zext(BTC)+1 when the BTC+1 computation was known not to overflow.

Differential Revision: https://reviews.llvm.org/D148661