* BIG CHANGE: The root of the call "graph" is considered to be the external
node, not the "Root". This means that users using graph iterators like
df_begin() will start from the external node.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@2957 91177308-0d34-0410-b5e6-96231b3b80d8