[Attributor] Ensure to use the proper liveness AA

When determining liveness via Attributor::isAssumedDead(...) we might
end up without a liveness AA or with one pointing into another function.
Neither is helpful and we will avoid both from now on.

Reapplied after fixing the ASAN error which caused the revert:
https://github.com/llvm/llvm-project/commit/db68a25ca90e0da46c9c33b027fa83260073bd28