[llvm] [WholeProgramDevirt] Add check for AvailableExternal and give up devirt (PR #143468)

[Teresa Johnson via llvm-commits]

https://github.com/teresajohnson commented:

Hoping @pcc will take a look, but a couple questions after skimming the changes.

Rather than completely shut off devirtualization, would it be better to simply suppress using a branch funnel? I.e. check the GV linkage types of the recorded target GVs in DevirtModule::tryICallBranchFunnel. 

Also, I noticed that the branch funnel intrinsic is not documented, so I don't fully understand what this instruction does (will ping https://github.com/llvm/llvm-project/issues/133635).

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