nir: change nir_lower_io_to_vector() so that it can always vectorize FS outputs
ACO needs there to be at most only one variable per target. With these changes, nir_lower_io_to_vector() should be able to guarantee that.
ACO needs there to be at most only one variable per target. With these changes, nir_lower_io_to_vector() should be able to guarantee that.