[gicombiner] Allow generated CombinerHelpers to have additional arguments

Summary:
This allows combiners to delegate to other helpers or depend
on additional information. It's not great as an overall
solution though as callers must provide the argument on every call, even for
static data like an additional helper. Another patch will follow to
support additional members of the generated combiner.

Reviewers: aditya_nandakumar, bogner, aemerson, paquette, volkan, arsenm

Reviewed By: aditya_nandakumar

Subscribers: wdng, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81862