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

lib/CodeGen/LatencyPriorityQueue.cpp

@@ -46,7 +46,7 @@ bool latency_sort::operator()(const SUnit *LHS, const SUnit *RHS) const {
 
   // Finally, just to provide a stable ordering, use the node number as a
   // deciding factor.
-  return LHSNum < RHSNum;
+  return RHSNum < LHSNum;
 }