[ScheduleDAG] Add statistics for maintaining the topological order.

[fhahn]

Summary

This is helpful to measure the impact of D60125 on maintaining
topological orders.

[efriedma]

LGTM

llvm/lib/CodeGen/ScheduleDAG.cpp

 #include "llvm/CodeGen/TargetInstrInfo.h"
 #include "llvm/CodeGen/TargetRegisterInfo.h"
 #include "llvm/CodeGen/TargetSubtargetInfo.h"
 #include "llvm/Config/llvm-config.h"
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/Compiler.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/raw_ostream.h"
+#include "llvm/ADT/Statistic.h"
 #include <algorithm>
 #include <cassert>
 #include <iterator>
 #include <limits>
 #include <utility>
 #include <vector>
 
 using namespace llvm;
 
 #define DEBUG_TYPE "pre-RA-sched"
 
+STATISTIC(NumNewPredsAdded, "Number of times a  single predecessor was added");
+STATISTIC(NumTopoInits, "Number of times the topological order has been recomputed");
+
 #ifndef NDEBUG
 static cl::opt<bool> StressSchedOpt(
   "stress-sched", cl::Hidden, cl::init(false),
   cl::desc("Stress test instruction scheduling"));
 #endif
 
 void SchedulingPriorityQueue::anchor() {}
 
for (const SDep &PredDep : SU->Preds) {
       if (SU->NodeNum < DAGSize && !--Node2Index[SU->NodeNum])
         // If all dependencies of the node are processed already,
         // then the node can be computed now.
         WorkList.push_back(SU);
     }
   }
 
   Visited.resize(DAGSize);
+  NumTopoInits++;
 
 #ifndef NDEBUG
   // Check correctness of the ordering
   for (SUnit &SU : SUnits)  {
     for (const SDep &PD : SU.Preds) {
       assert(Node2Index[SU.NodeNum] > Node2Index[PD.getSUnit()->NodeNum] &&
       "Wrong topological sorting");
     }
void ScheduleDAGTopologicalSort::AddPred(SUnit *Y, SUnit *X) {
   if (LowerBound < UpperBound) {
     // Update the topological order.
     Visited.reset();
     DFS(Y, UpperBound, HasLoop);
     assert(!HasLoop && "Inserted edge creates a loop!");
     // Recompute topological indexes.
     Shift(Visited, LowerBound, UpperBound);
   }
+
+  NumNewPredsAdded++;
 }
 
 void ScheduleDAGTopologicalSort::RemovePred(SUnit *M, SUnit *N) {
   // InitDAGTopologicalSorting();
 }
 
 void ScheduleDAGTopologicalSort::DFS(const SUnit *SU, int UpperBound,
                                      bool &HasLoop) {