normalize nary checks in fm_poison

[tnuha]

This was the biggest difference I was referencing in #1095 (comment). Ultimately I'd like #1095 to be a smooth merge if they beat me to it since the work is pretty solid; without this change, and assuming a simple merge, there's a de-referenced null pointer issue that comes about. I can track down some examples if desired.

[nunoplopes]

There's no dereference of a null pointer. A null expression just propagates. Can you find an example that exhibits a bug?

[tnuha]

Yeah, sorry, I meant there was an assertion failure due to a null pointer propagation. This eventually crashes alive2 in certain cases like with a naive merge of master into #1095's current branch. e.g. alive-tv --uf-float tests/alive-tv/uf-float/add-comm-double.srctgt.ll has the attached report on this commit (an immediate merge of master into emmtrix's branch) but not this one.

report.txt

To my knowledge this doesn't currently hit any bugs on master, it just seems like a necessary change as it's a current blocker for at least that branch, and it may inhibit some other work in the future. Totally understand any considerations that marks this as currently unnecessary, though.