i965: Consign COORD_REPLACE VS hacks to Pre-Gen6.

Pre-Gen6, the SF thread requires exact matching between VS output
slots (aka VUE slots) and FS input slots, even when the corresponding
VS output slot is unused due to being overwritten by point coordinate
replacement (glTexEnvi(GL_POINT_SPRITE, GL_COORD_REPLACE, GL_TRUE)).
As a result, we have a special hack in the VS to ensure when any
texture coordinate is subject to point coordinate replacement, it is
always allocated space in the VUE, even if it isn't written to by the
VS.

This hack isn't needed from Gen6 onwards, since SF (Gen7: SBE)
swizzling has the ability to insert the point coordinate into
gl_TexCoord[] without needing a corresponding unused VUE slot.

Note that no modification of SF setup code is required for this
patch--get_attr_override() already does the right thing.  However, we
make a slight comment change to clarify why this works.

In addition to eliminating unnecessary VS recompiles and saving
precious URB space on Gen6+, this will save us the trouble of having
to adjust this hack when we implement geometry shaders.
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: make vec4_visitor::vp more generic
WIP: clean up uses of prog_data.
WIP: split brw_vs_compile out to brw_vec4_compile
WIP: split brw_vs_prog_key
WIP: split brw_vs_prog_data
WIP: make vs_vec4_visitor class
WIP: extract ir_var_system_value handling from ir_variable visitor
WIP: move run() to vs_vec4_visitor
WIP: move emit_vertex_program_code() to vs_vec4_visitor
WIP: move setup_vp_regs() to vs_vec4_visitor
WIP: move get_vp_src_reg() to vs_vec4_visitor
WIP: move emit_attribute_fixups() to vs_vec4_visitor
WIP: move emit_urb_writes() to vs_vec4_visitor
WIP: move emit_urb_slot() to vs_vec4_visitor
WIP: move emit_generic_urb_slot() to vs_vec4_visitor
WIP: move setup_attributes() to vs_vec4_visitor
WIP: move emit_psiz_and_flags() to vs_vec4_visitor
WIP: move emit_clip_distances() to vs_vec4_visitor