Fix the "partial pool allocator" on em3d and others. The problem is that

Fix the "partial pool allocator" on em3d and others.  The problem is that
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