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
Move all of the reusable logic out of `GlobalISelEmitter.cpp` into a `GlobalISelMatchTableExecutorEmitter` class so the future combiner backend can use it as well.
Depends on D153755
Reviewed By: aemerson
Differential Revision: https://reviews.llvm.org/D153756
