[RFC][TableGen][GlobalISel] Add Combiner Match Table Backend

Adds a new backend to power the GISel Combiners using the InstructionSelector's match tables.
This does not depend on any of the data structures created for the current combiner and is intended to replace it entirely.

See the RFC for more details: https://discourse.llvm.org/t/rfc-matchtable-based-globalisel-combiners/71457/6
Note: this would replace D141135.

Reviewed By: aemerson, arsenm

Differential Revision: https://reviews.llvm.org/D153757