[lworld] Extend loading of signature classes to flattened fields

`Method::load_signature_classes` forces loading of all argument types before compilation. We should evaluate if this needs to be extended to arguments that were not declared but only brought in through flattening (calling convention optimizations). We would then also need to adapt `Method::has_unloaded_classes_in_signature`.

The following assert catches such unloaded arguments:

collect_inline_fields(ciInlineKlass* vk, const Type** field_array, u
     ciField* field = vk->nonstatic_field_at(j);
     BasicType bt = field->type()->basic_type();
     const Type* ft = Type::get_const_type(field->type());
+#ifdef ASSERT
+ if (ft->isa_instptr() != NULL && !ft->is_instptr()->is_loaded()) {
+ ft->dump_on(tty);
+ assert(false, "Signature class not loaded");
+ }
+#endif