st/mesa: Use prog_to_nir for ARB vertex/fragment shaders and FF VS
This series switches st/mesa to use prog_to_nir for ARB programs and FF VS programs, for drivers which prefer NIR, rather than providing drivers TGSI shaders. This had landed briefly in tree a year ago, but had a number of problems and was ultimately reverted. I've since fixed up a whole lot of issues, and I believe it should be good to go this time.