ADT: Guarantee transferNodesFromList is only called on transfers

Guarantee that ilist_traits<T>::transferNodesFromList is only called
when nodes are actually changing lists.

I also moved all the callbacks to occur *first*, before the operation.
This is the only choice for iplist<T>::merge, so we might as well be
consistent.  I expect this to have no effect in practice, although it
simplifies the logic in both iplist<T>::transfer and iplist<T>::insert.

llvm-svn: 280122