[llvm] [FuncSpec] Skip SCCP on blocks of dead functions and poison their callsites (PR #154668)

[Alexandros Lamprineas via llvm-commits]

https://github.com/labrinea approved this pull request.

I think I am satisfied with the patch in its current form. If I am missing something I hope that a bootstrap build will catch it for us.

https://github.com/llvm/llvm-project/pull/154668