[Pipeliner] Fix the Schedule DAG topoligical order.

[sgundapa]

Summary

This patch updates the DAG change to reflect in the topological ordering
of the nodes.

[sgundapa]

I was not able to formulate a unit test that exposes this issue.
I figured this problem when I was working on a DAG mutation for the pipeliner.

llvm/trunk/lib/CodeGen/MachinePipeliner.cpp

for (SUnit &I : SUnits) {
     for (int i = 0, e = Deps.size(); i != e; i++) {
       Topo.RemovePred(LastSU, Deps[i].getSUnit());
       LastSU->removePred(Deps[i]);
     }
 
     // Add a dependence between the new instruction and the instruction
     // that defines the new base.
     SDep Dep(&I, SDep::Anti, NewBase);
+    Topo.AddPred(LastSU, &I);
     LastSU->addPred(Dep);
 
     // Remember the base and offset information so that we can update the
     // instruction during code generation.
     InstrChanges[&I] = std::make_pair(NewBase, NewOffset);
   }
 }