[Attributor] Don't crash if getAnalysisResultForFunction() returns null LoopInfo

I have no idea what's going on here. This code was moved
around/introduced in change cb26b01d and starts crashing with a NULL
dereference once I apply https://reviews.llvm.org/D123090. I assume that
I've unwittingly taught the attributor enough that it's able to do more
clever things than in the past, and it's able to trip on this case. I
make no claims about the correctness of this patch, but it passes tests
and seems to fix all the crashes I've been seeing.

Differential Revision: https://reviews.llvm.org/D129589