[SCEV] Further clarify comments regarding UB and zero stride

Follow on to D109029. I realized we had no mention of mustprogrress in the comment (as it prexisted mustprogress in the codebase). In the process of adding it, I tweaked the preconditions into something I think is more clear. Note that mustprogress is checked in the code.

Differential Revision: https://reviews.llvm.org/D109091