ScheduleTree: make type_ a const member

This field is used in multiple static dispatches and is publicly
exposed.  It is unclear why it has not been const from the start.  The
last assignments to it were removed in the previous commit.  Make it a
const member assignable only in constructors.

Author: ftynse

tc/core/polyhedral/schedule_tree.h

@@ -477,7 +477,7 @@ struct ScheduleTree {
   std::vector<ScheduleTreeUPtr> children_{};
 
  public:
-  detail::ScheduleTreeType type_{detail::ScheduleTreeType::None};
+  const detail::ScheduleTreeType type_{detail::ScheduleTreeType::None};
 };
 
 // Flatten nested nodes of the same type.