v3dv: nir optimization cleanup
In spite of the name, and the last patch, the main objective for this MR is to fix the following test:
dEQP-VK.graphicsfuzz.cov-loop-condition-clamp-vec-of-ones
That has been haunting us for a while.
As part of the optimization cleanup, the shaderdb with our usual Vulkan test applications get slightly worse on some statistics. But there is not noticiable performance drop.