i965: Save the gl_shader_stage enum in backend_visitor.

i965: Save the gl_shader_stage enum in backend_visitor.

This will be useful for INTEL_DEBUG=optimizer in the vec4 backend, which
needs to know whether it's currently processing a VS or GS.  It isn't
worth adding virtual methods for this case.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>