Add detailed comment.

Author: tianleliu

llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp

@@ -1456,7 +1456,17 @@ void DevirtModule::tryICallBranchFunnel(
   if (!HasNonDevirt)
     return;
 
-  // If any GV is AvailableExternally, drop to generate branch.funnel
+  // If any GV is AvailableExternally, not to generate branch.funnel.
+  // NOTE: It is to avoid crash in LowerTypeTest.
+  // If the branch.funnel is generated, because GV.isDeclarationForLinker(),
+  // in LowerTypeTestsModule::lower(), its GlobalTypeMember would NOT
+  // be saved in GlobalTypeMembers[&GV]. Then crash happens in
+  // buildBitSetsFromDisjointSet due to GlobalTypeMembers[&GV] is NULL.
+  // Even doing experiment to save it in GlobalTypeMembers[&GV] and
+  // making GlobalTypeMembers[&GV] be not NULL, crash could avoid from
+  // buildBitSetsFromDisjointSet. But still report_fatal_error in Verifier
+  // or SelectionDAGBuilder later, because operands linkage type consistency
+  // check of icall.branch.funnel can not pass.
   for (auto &T : TargetsForSlot) {
     if (T.TM->Bits->GV->hasAvailableExternallyLinkage())
       return;