[InstCombine] Negator: while there, add detection for cycles during negation

I don't have any testcases showing it happening,
and i haven't succeeded in creating one,
but i'm also not positive it can't ever happen,
and i recall having something that looked like
that in the very beginning of Negator creation.

But since we now already have a negation cache,
we can now detect such cases practically for free.

Let's do so instead of "relying" on stack overflow :D