[SCEV] Generalize getSmallConstantTripCount(L) for multiple exit loops

This came up in review for another patch, see https://reviews.llvm.org/D102982#2782407 for full context.

I've reviewed the callers to make sure they can handle multiple exit loops w/non-zero returns.  There's two cases in target cost models where results might change (Hexagon and PowerPC), but the results looked legal and reasonable.  If a target maintainer wishes to back out the effect of the costing change, they should explicitly check for multiple exit loops and handle them as desired.

Differential Revision: https://reviews.llvm.org/D103182