[AMDGPU] Use GlobalISel MatchTable Combiner Backend

Use the new matchtable-based combiner backend for all AMDGPU combiners.
This drop-in from the user's perspective; there are no test changes, the new combiner behaves exactly like the old one.

Depends on D153757

NOTE: This would land iff D153757 (RFC) lands too.

Reviewed By: arsenm

Differential Revision: https://reviews.llvm.org/D153758