[Attributor] Use existing function information for the call site

Summary:
Instead of recomputing information for call sites we now use the
function information directly. This is always valid and once we have
call site specific information we can improve here.

This patch also bootstraps attributes that are created on-demand through
an initial update call. Information that is known will then directly be
available in the new attribute without causing an iteration delay.

The tests show how this improves the iteration count.

Reviewers: sstefan1, uenoku

Subscribers: hiraditya, bollu, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66781

llvm-svn: 370480