i965/vs: Move vs-specific code out of brw_vec4.h.

Now brw_vec4.h contains only code that is shared between the vertex
and geometry shaders.
i965/vec4: Make with_writemask() non-static.

This will allow it to be shared between brw_vec4_visitor.cpp and
brw_vec4_vs_visitor.cpp (which will be created in the next patch).
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.