[SCEV] Clarify requirements for zero-stride to be UB

[SCEV] Clarify requirements for zero-stride to be UB

There's a silent bug in our reasoning about zero strides. We assume that having a single static exit implies that if that exit is not taken, then the loop must be infinite. This ignores the potential for abnormal exits via exceptions. Consider the following example:

for (uint_8 i = 0; i < 1; i += 0) {
  throw_on_thousandth_call();
}

Our reasoning is such that we'd conclude this loop can't take the backedge as that would lead to a (presumed) infinite loop.

In practice, this is a silent bug because the loopIsFiniteByAssumption returns false strictly more often than the loopHaNoAbnormalExits property. We could reasonable want to change that in the future, so fixing the codeflow now is worthwhile.

Differential Revision: https://reviews.llvm.org/D109029