r600-nir: Add support for dvec4 or lower them to 2x dvec2
soft fp64 uses dvec4, so either handle this in the backend, or write a nir pass that lowers them to two dvec2 that can be handled normally.
soft fp64 uses dvec4, so either handle this in the backend, or write a nir pass that lowers them to two dvec2 that can be handled normally.