glsl: Create a _mesa_shader_stage_to_abbrev() function.

glsl: Create a _mesa_shader_stage_to_abbrev() function.

This is similar to _mesa_shader_stage_to_string(), but returns "VS"
instead of "vertex".

v2: Use unreachable() and add MESA_SHADER_COMPUTE (requested by Ian).

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>