[ScalarEvolution] Fix overflow in computeBECount.

The current implementation of computeBECount doesn't account for the
possibility that adding "Stride - 1" to Delta might overflow. For almost
all loops, it doesn't, but it's not actually proven anywhere.

To deal with this, use a variety of tricks to try to prove that the
addition doesn't overflow.  If the proof is impossible, use an alternate
sequence which never overflows.

Differential Revision: https://reviews.llvm.org/D105216