[Attributor] AAFunctionReachability, Instruction reachability.

This patch implement instruction reachability for AAFunctionReachability
attribute. It is used to tell if a certain instruction can reach a function
transitively.

NOTE: I created a new commit based of D106720 and set the author back to
      Kuter. Other metadata, etc. is wrong. I also addressed the
      remaining review comments and fixed the unit test.

Differential Revision: https://reviews.llvm.org/D106720