glsl: Set ir_instruction::ir_type in the base class constructor

glsl: Set ir_instruction::ir_type in the base class constructor

This has the added perk that if you forget to set ir_type in the
constructor of a new subclass (or a new constructor of an existing
subclass) the compiler will tell you... instead of relying on
ir_validate or similar run-time detection.

Reviewed-by: Juha-Pekka Heikkila <juhapekka.heikkila@gmail.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>