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

[Tianle Liu via llvm-commits]

tianleliu wrote:

> I would think it would also fix split-LTO-unit ThinLTO mode (e.g. "-flto=thin -fsplit-lto-unit" which splits the module into ThinLTO and regular LTO sub modules - is that not the case?)

Yes. The patch also fixes crash for "-flto=thin -fsplit-lto-unit" mode.

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