Don't serialize redefined symbol nodes (#7499)
Redefined nodes sometimes have busted internal references (like a self
argument that has the class as its type even though it won't be able
to look it up) which can cause crashes. Since they can't have
references from outside themselves, we don't lose anything by not
serializing them.
Fixes a crash I observed at Dropbox.
This partially reverts #7413, which introduced this crash (while
fixing another), but leaves the test case it added.
