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

[durin42]

Summary

I have no idea what's going on here. This code was moved
around/introduced in change cb26b01d57f5 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.

[jdoerfert]

LG with return true.

llvm/lib/Transforms/IPO/AttributorAttributes.cpp

6406

return true is the conservative answer here.

llvm/lib/Transforms/IPO/AttributorAttributes.cpp

ChangeStatus AAHeapToStackFunction::updateImpl(Attributor &A) {
   Optional<bool> MayContainIrreducibleControl;
   auto IsInLoop = [&](BasicBlock &BB) {
     if (&F->getEntryBlock() == &BB)
       return false;
     if (!MayContainIrreducibleControl.has_value())
       MayContainIrreducibleControl = mayContainIrreducibleControl(*F, LI);
     if (MayContainIrreducibleControl.value())
       return true;
+    if (!LI)
+      return true;
     return LI->getLoopFor(&BB) != nullptr;
   };
 
   // Flag to ensure we update our deallocation information at most once per
   // updateImpl call and only if we use the free check reasoning.
   bool HasUpdatedFrees = false;
 
   auto UpdateFrees = [&]() {