Initialize SUnits before DAG building.
Affect on SD scheduling and postRA scheduling:
Printing the DAG will display the nodes in top-down topological order.
This matches the order within the MBB and makes my life much easier in general.
Affect on misched:
We don't need to track virtual register uses at all. This is awesome.
I also intend to rely on the SUnit ID as a topo-sort index. So if A < B then we cannot have an edge B -> A.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@151135
91177308-0d34-0410-b5e6-
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