[GlobalISel] Refactor Combiner API

Remove CodeGen leftovers from the old combiner backend and adapt the API to fit the new backend better.
It's now quite a bit closer to how InstructionSelector works.

- `CombinerInfo` is now a simple "options" struct.
- `Combiner` is now the base class of all TableGen'd combiner implementation.
    - Many fields have been moved from derived classes into that class.
    - It has been refactored to create & own the Observer and Builder.
- `tryCombineAll` TableGen'd method can now be renamed, which allows targets to implement the actual `tryCombineAll` call manually and do whatever they want to do before/after it.

Note: `CombinerHelper` needs to be mutable because none of its methods are const. This can be revisited later.

Depends on D158710

Reviewed By: aemerson, dsanders

Differential Revision: https://reviews.llvm.org/D158713