Change the 'global modification' APIs in SelectionDAG to take a new

DAGUpdateListener object pointer instead of just returning a vector 
of deleted nodes.  This makes the interfaces more efficient (no more
allocating a vector [at least a malloc], filling it in, then walking
it) and more clean.  This also allows the client to be notified of
nodes that are *changed* but not deleted.

llvm-svn: 46677