Invoke DAG postprocessing in the post-RA scheduler

This was inadvertently omitted from r262774, which added the mutation
interface.

llvm-svn: 262939

llvm/lib/CodeGen/PostRASchedulerList.cpp

@@ -405,6 +405,8 @@ void SchedulePostRATDList::schedule() {
     }
   }
 
+  postprocessDAG();
+
   DEBUG(dbgs() << "********** List Scheduling **********\n");
   DEBUG(
     for (const SUnit &SU : SUnits) {