[Attributor] Liveness analysis.

Liveness analysis abstract attribute used to indicate which BasicBlocks are dead and can therefore be ignored.
Right now we are only looking at noreturn calls.

Reviewers: jdoerfert, uenoku

Subscribers: hiraditya, llvm-commits

Differential revision: https://reviews.llvm.org/D64162

llvm-svn: 366736