[gicombiner] Allow generated combiners to store additional members

Summary:
Adds the ability to add members to a generated combiner via
a State base class. In the current AArch64PreLegalizerCombiner
this is used to make Helper available without having to
provide it to every call.

As part of this, split the command line processing into a
separate object so that it still only runs once even though
the generated combiner is constructed more frequently.

Depends on D81862

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

Reviewed By: arsenm

Subscribers: jvesely, wdng, nhaehnle, kristof.beyls, hiraditya, kerbowa, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81863