glsl: Move is_built_in flag from ir_function_signature to ir_function.

Also rename it to "is_builtin" for consistency.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>

src/glsl/ir_constant_expression.cpp

@@ -785,7 +785,7 @@ ir_call::constant_expression_value()
     * "Function calls to user-defined functions (non-built-in functions)
     *  cannot be used to form constant expressions."
     */
-   if (!this->callee->is_built_in)
+   if (!this->callee->function()->is_builtin)
       return NULL;
 
    unsigned num_parameters = 0;