[Attributor][Fix] Resolve various liveness issues

Summary:
This contains various fixes:
  - Explicitly determine and return the next noreturn instruction.
  - If an invoke calls a noreturn function which is not nounwind we
    keep the unwind destination live. This also means we require an
    invoke. Though we can still add the unreachable to the normal
    destination block.
  - Check if the return instructions are dead after we look for calls
    to avoid triggering an optimistic fixpoint in the presence of
    assumed liveness information.
  - Make the interface work with "const" pointers.
  - Some simplifications

While additional tests are included, full coverage is achieved only with
D59978.

Reviewers: sstefan1, uenoku

Subscribers: hiraditya, bollu, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65701

llvm-svn: 367791