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

[Teresa Johnson via llvm-commits]

https://github.com/teresajohnson commented:

Code change and test look good. In the absence of a review from someone more familiar with this construct, this seems like the right approach to at least work around crashes in LTT. I have a few questions and suggested fixes in the code comment and the PR description for readability and correctness.

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