[SCEV] Handle zero stride correctly in howManyLessThans

This is split from D105216, but the code is hoisted much earlier into
the path where we can actually get a zero stride flowing through. Some
fairly simple proofs handle the cases which show up in practice. The
only test changes are the cases where we really do need a non-zero
divider to produce the right result.

Recommitting with isLoopInvariant() check.

Differential Revision: https://reviews.llvm.org/D105921