[gicombiner] Hoist pure C++ combine into the tablegen definition

Summary:
This is just moving the existing C++ code around and will be NFC w.r.t
AArch64. Renamed 'CombineBr' to something more descriptive
('ElideByByInvertingCond') at the same time.

The remaining combines in AArch64PreLegalizeCombiner require features that
aren't implemented at this point and will be hoisted as they are added.

Depends on D68424

Reviewers: bogner, volkan

Subscribers: kristof.beyls, hiraditya, Petar.Avramovic, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68426

llvm-svn: 375057