glsl: Convert ir_function_signature::is_builtin to a method.

glsl: Convert ir_function_signature::is_builtin to a method.

A signature is a built-in if and only if builtin_info != NULL, so we
don't actually need a separate flag bit.  Making a boolean-valued
method allows existing code to ask the same question while not worrying
about the internal representation.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Reviewed-by: Paul Berry <stereotype441@gmail.com>