[Attributor] Restrict liveness and return information to functions

Summary:
Until we have proper call-site information we should not recompute
liveness and return information for each call site. This patch directly
uses the function versions and introduces TODOs at the usage sites.

The required iterations to get to the fixpoint are most of the time
reduced by this change and we always avoid work duplication.

Reviewers: sstefan1, uenoku

Subscribers: hiraditya, bollu, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66562

llvm-svn: 370208