[utils] Refactor utils/update_{,llc_}test_checks.py to share more code

Summary:
This revision refactors 1. parser 2. CHECK line adder of utils/update_{,llc_}test_checks.py
so that thir functionality can be re-used by other utility scripts (e.g.  D42712)

Reviewers: asb, craig.topper, RKSimon, echristo

Subscribers: llvm-commits, spatel

Differential Revision: https://reviews.llvm.org/D42805

llvm-svn: 324803