...it avoids false warnings about unchecked dynamic_cast results from static
code analyzers, and potentially makes the code a little faster, too.  (Most of
these dynamic_casts were kind of a leftover from a very early design that
dispateched on Node type via dynamic_cast instead of a Node::Kind, but which was
much too slow.)

Change-Id: I8db08a1c35783f8ea6c51eed55b11faa2d958807