[Attributor] Split the Attributor::run() into multiple functions.

Summary:
This patch splits the Attributor::run() function into multiple functions.

Simple Logic changes to make this possible:
  # Moved iteration count verification earlier.
  # NumFinalAAs get set a little bit later.

Reviewers: jdoerfert, sstefan1, uenoku

Reviewed By: jdoerfert

Subscribers: hiraditya, uenoku, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81022