i965/vs: Move vs-specific code out of brw_vec4_visitor.cpp.

i965/vs: Move vs-specific code out of brw_vec4_visitor.cpp.

This patch creates a new file brw_vec4_vs_visitor.cpp, to contain code
that is specific to the vertex shader.  Now the organization of vertex
shader and geometry shader visitor code is symmetric: vs-specific code
is in brw_vec4_vs_visitor.cpp, gs-specific code is in
brw_vec4_gs_visitor.cpp, and code shared between vs and gs is in
brw_vec4_visitor.cpp.

Acked-by: Kenneth Graunke <kenneth@whitecape.org>