i965/vs: Generalize attribute setup code in preparation for GS.

i965/vs: Generalize attribute setup code in preparation for GS.

This patch introduces a new function,
vec4_visitor::lower_attributes_to_hw_regs(), which replaces registers
of type ATTR in the instruction stream with the hardware registers
that store those attributes.  This logic will need to be common
between the vertex and geometry shaders.

Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>