DSNodes, unlike other GraphTraits nodes, can have null outgoing edges, and
df_iterator doesn't take this into consideration. As a workaround, the
successor iterator now handles null nodes and 'indicates' that null has
no successors.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@12025 91177308-0d34-0410-b5e6-96231b3b80d8