Fix not short-circuiting null coalescing operator

Related to #534 (closed), #640 (closed).

This is not a complete solution, but in almost all usual cases, short-circuiting null coalescing operator works well. Assigning local_type.value_owned = true just add one-step memory copy, and when experimental_non_null is not activated, evaluating the type of right child node earlier is not essential for type inference.