[SCEV] Clarify inference in isAddRecNeverPoison()

The justification in isAddRecNeverPoison() no longer applies, as
it dates back to a time where LLVM had an unconditional forward
progress guarantee. However, we also no longer need it, because we
can exploit branch on poison UB instead.

For a single exit loop (without abnormal exits) we know that all
instructions dominating the exit will be executed, so if any of
them trigger UB on poison that means that addrec is not poison.

This is slightly stronger than the previous code, because a) we
don't need the exit to also be the latch and b) we don't need the
value to be used in the exit branch in particular, any UB-producing
instruction is fine.

I don't expect much practical impact from this change, this is
mainly to clarify the reasoning behind this logic.

Differential Revision: https://reviews.llvm.org/D148633