glsl/ir: add subroutine information storage to ir_function (v1.1)

We need to store two sets of info into the ir_function,
if this is a function definition with a subroutine list
(subroutine_def) or if it a subroutine prototype.

v1.1: add some more documentation.

Reviewed-by: Chris Forbes <chrisf@ijw.co.nz>
Signed-off-by: Dave Airlie <airlied@redhat.com>

src/glsl/ir_print_visitor.cpp

@@ -231,7 +231,7 @@ void ir_print_visitor::visit(ir_function_signature *ir)
 
 void ir_print_visitor::visit(ir_function *ir)
 {
-   fprintf(f, "(function %s\n", ir->name);
+   fprintf(f, "(%s function %s\n", ir->is_subroutine ? "subroutine" : "", ir->name);
    indentation++;
    foreach_in_list(ir_function_signature, sig, &ir->signatures) {
       indent();