[RFC] MatchTable-based GlobalISel Combiners

[Pierre-vh]

I posted a first series of patches.
I’ll write & post the remaining patches (AArch64 port, misc ports, delete old combiner) in the coming days/week.

• ⚙ D153755 [GlobalISel] Generalize `InstructionSelector` Match Tables
• ⚙ D153756 [TableGen] Add Generic MatchTableExecutor Emitter
• ⚙ D153757 [RFC][TableGen][GlobalISel] Add Combiner Match Table Backend
• ⚙ D153758 [AMDGPU] Use GlobalISel MatchTable Combiner Backend