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.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@46677 91177308-0d34-0410-b5e6-96231b3b80d8
