glsl: Clarify ir_function::matching_sigature()

glsl: Clarify ir_function::matching_sigature()

The function used a variable named 'score', which was an outright lie.
A signature matches or it doesn't; there is no fuzzy scoring.

Change the return type of parameter_lists_match() to an enum, and
let ir_function::matching_sigature() switch on that enum.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Signed-off-by: Chad Versace <chad@chad-versace.us>