WIP: split brw_vs_prog_data
WIP: move setup_attributes() to vs_vec4_visitor
WIP: extract ir_var_system_value handling from ir_variable visitor
WIP: move emit_psiz_and_flags() to vs_vec4_visitor
WIP: move emit_clip_distances() to vs_vec4_visitor
WIP: move prog_data to vs_vec4_visitor