glsl: Move is_builtin flag back to ir_function_signature.

This effectively reverts b6f15869b3.

In desktop GLSL, defining a function with the same name as a built-in
hides that built-in function completely, so there would never be
built-in and user function signatures in the same ir_function.

However, in GLSL ES, overloading built-ins is allowed, and does not
hide the built-in signatures - so we're back to needing this.

src/glsl/ir_print_visitor.cpp

@@ -153,15 +153,14 @@ void ir_print_visitor::visit(ir_function_signature *ir)
 
 void ir_print_visitor::visit(ir_function *ir)
 {
-   /* Don't print built-in functions as part of the IR. */
-   if (ir->is_builtin)
-      return;
-
    printf("(function %s\n", ir->name);
    indentation++;
    foreach_iter(exec_list_iterator, iter, *ir) {
       ir_function_signature *const sig = (ir_function_signature *) iter.get();
 
+      if (sig->is_builtin)
+	 continue;
+
       indent();
       sig->accept(this);
       printf("\n");