[SCEV] Compute trip multiple for multiple exit loops

This patch implements getSmallConstantTripMultiple(L) correctly for multiple exit loops. The previous implementation was both imprecise, and violated the specified behavior of the method. This was fine in practice, because it turns out the function was both dead in real code, and not tested for the multiple exit case.

Differential Revision: https://reviews.llvm.org/D103189