Mesa's Radeon R600 Gallium3D Driver Now Has NIR Support Under Review
The R600g NIR support has been worked on by Gert Wollny and currently targets Radeon HD 5000 "Evergreen" graphics cards with support for other AMD GPU generations handled by this Gallium3D driver not yet supported. Additionally, this NIR back-end only supports vertex / fragment / geometry shaders for now and other features missing.
The preliminary support can be found via this merge request. This R600g NIR support would be disabled by default as a lot of features and optimizations are left to be tackled, but for those interested it's one step closer to mainline for those using these old pren-GCN Radeon graphics cards.