WIP: clean up uses of prog_data.
WIP: move c to vs_vec4_visitor.
i965: Remove unused userclip flags.

brw_vs_prog_data::userclip hasn't been used since commit f0cecd4
(i965: Move VUE map computation to once at VS compile time).

brw_gs_prog_key::userclip_active hasn't been used since commit 9f3d321
(i965: Make the userclip flag for the VUE map come from VS prog data).
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
WIP: move get_vp_dst_reg() to vs_vec4_visitor
WIP: move vp_temp_regs to vs_vec4_visitor
WIP: move vp_addr_reg to vs_vec4_visitor
WIP: move emit_ndc_computation() to vs_vec4_visitor