[JumpThreading][VectorUtils] avoid infinite loop on unreachable IR

https://llvm.org/PR48362

It's possible that we could stub this out sooner somewhere
within JumpThreading, but I'm not sure how to do that, and
then we would still have potential danger in other callers.

I can't find a way to trigger this using 'instsimplify',
however, because that already has a bailout on unreachable
blocks.