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-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/CriticalAntiDepBreaker.cpp b/lib/CodeGen/CriticalAntiDepBreaker.cpp
index 5c32539..0751229 100644
--- a/lib/CodeGen/CriticalAntiDepBreaker.cpp
+++ b/lib/CodeGen/CriticalAntiDepBreaker.cpp
@@ -427,6 +427,8 @@
 
   // Keep a map of the MachineInstr*'s back to the SUnit representing them.
   // This is used for updating debug information.
+  //
+  // FIXME: Replace this with the existing map in ScheduleDAGInstrs::MISUnitMap
   DenseMap<MachineInstr*,const SUnit*> MISUnitMap;
 
   // Find the node at the bottom of the critical path.