i965: Create backend_visitor fields for debugging messages.

We introduce three new fields in backend_visitor:
- debug_enabled: whether or not INTEL_DEBUG & DEBUG_<stage flag>
- stage_name: "vertex", "fragment", etc. for use in messages
- stage_abbrev: "VS", "FS", etc. for use in messages

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Jason Ekstrand <jason.ekstrand@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kristian Høgsberg <krh@bitplanet.net>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>

src/mesa/drivers/dri/i965/brw_shader.cpp

      cfg(NULL),
      stage(stage)
 {
+   debug_enabled = INTEL_DEBUG & intel_debug_flag_for_shader_stage(stage);
+   stage_name = _mesa_shader_stage_to_string(stage);
+   stage_abbrev = _mesa_shader_stage_to_abbrev(stage);
 }
 
 bool

src/mesa/drivers/dri/i965/brw_shader.h

    cfg_t *cfg;
 
    gl_shader_stage stage;
+   bool debug_enabled;
+   const char *stage_name;
+   const char *stage_abbrev;
 
    brw::simple_allocator alloc;