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

[via llvm-commits]

XChy wrote:

It seems that (post) dom tree analysis is not updated. I am still trying to compile LLVM with EXPENSIVE_CHECKS. Hopefully I will post the fix today.
@mikaelholmen, feel free to revert if it's emergent.

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