[LCG] We don't actually need a set in each SCC to track the nodes. We
can use the node -> SCC mapping in the top-level graph to test this on
the rare occasions we need it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207090 91177308-0d34-0410-b5e6-96231b3b80d8