[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 cb26b01d57 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

llvm/lib/Transforms/IPO/AttributorAttributes.cpp

@@ -6402,6 +6402,8 @@ ChangeStatus AAHeapToStackFunction::updateImpl(Attributor &A) {
       MayContainIrreducibleControl = mayContainIrreducibleControl(*F, LI);
     if (MayContainIrreducibleControl.value())
       return true;
+    if (!LI)
+      return true;
     return LI->getLoopFor(&BB) != nullptr;
   };