[gicombiner] Add the boring boilerplate for the declarative combiner

Summary:
This is the first of a series of patches extracted from a much bigger WIP
patch. It merely establishes the tblgen pass and the way empty combiner
helpers are declared and integrated into a combiner info.

The tablegen pass takes a -combiners option to select the combiner helper
that will be generated. This can be given multiple values to generate
multiple combiner helpers at once. Doing so helps to minimize parsing
overhead.

The reason for creating a GlobalISel subdirectory in utils/TableGen is that
there will be quite a lot of non-pass files (~15) by the time the patch
series is done.

Reviewers: volkan

Subscribers: mgorny, hiraditya, simoncook, Petar.Avramovic, s.egerton, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68286

llvm-svn: 373527