[llvm] [WholeProgramDevirt] Add check for AvailableExternal and give up icall.branch.funnel (PR #143468)

[Teresa Johnson via llvm-commits]

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@@ -1456,6 +1456,12 @@ void DevirtModule::tryICallBranchFunnel(
   if (!HasNonDevirt)
     return;
 
+  // If any GV is AvailableExternally, drop to generate branch.funnel
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teresajohnson wrote:

Change "drop to generate" to "do not generate". Also, please add a note that this avoids a crash in LowerTypeTest due to the GV being dropped resulting in a null GlobalTypeMember.

https://github.com/llvm/llvm-project/pull/143468