RADV's Vulkan Ray-Tracing LBVH Extended Back To All GCN GPUs

Mesa's Radeon Vulkan "RADV" driver is in the unique position of supporting Vulkan ray-tracing for older AMD GPUs rather than just the latest-generation RDNA2 GPUs with dedicated ray-tracing cores. Though it's slower on these older GPUs, the code is in place for this open-source driver and the latest addition is now supporting LBVH going back to AMD GFX6 hardware -- in other words, all GCN GPUs...