Builds on top of 6de2735c2428 to fix remaining issues with iteration order in the MatchTable Combiner backend.
See D155789 as well.
Reviewed By: MaskRay
Differential Revision: https://reviews.llvm.org/D155821
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