DSGraphs while they are forwarding. When the last reference to the forwarding
node is dropped, the forwarding node is autodeleted. This should simplify
removeTriviallyDead nodes, and is only (efficiently) possible because we are
using an ilist of dsnodes now.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@11175 91177308-0d34-0410-b5e6-96231b3b80d8