The MatchTable-based GlobalISel Combiner backend is the new default. There are no in-tree users left of the old backend.
- Removed implementation of old MatchDAG-based Combiner, including tests, the backend itself and all supporting code.
- Renamed MatchTable backend to GlobalISelCombinerEmitter.cpp + removed "-matchtable" from its CL option.
- no need to have a verbose name as it's the only backend left now.