[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
[LCG] Switch the parent SCC tracking from a SmallSetVector to
a SmallPtrSet. Currently, there is no need for stable iteration in this
dimension, and I now thing there won't need to be going forward.

If this is ever re-introduced in any form, it needs to not be
a SetVector based solution because removal cannot be linear. There will
be many SCCs with large numbers of parents. When encountering these, the
incremental SCC update for intra-SCC edge removal was quadratic due to
linear removal (kind of).

I'm really hoping we can avoid having an ordering property here at all
though...

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207091 91177308-0d34-0410-b5e6-96231b3b80d8