Rewritten R600g NIR Backend Merged For Mesa 22.2 - Improves Radeon HD 5000/6000 Series

The rewritten NIR back-end for the R600 Gallium3D driver with a focus on improving performance for old Radeon HD 5000/6000 series hardware has been merged! This also allows proper FP64 usage finally for the Radeon HD 6900 "Cayman" series graphics cards...