Add a policy to enable computing SchedDFSResult.

[rudkx]

Summary

Make GenericScheduler compute SchedDFSResult on initialization if
the policy is set. This makes it possible to create classes
that extend GenericScheduler and rely on the results of SchedDFSResult,
e.g. to perform subtree scheduling.

NFC unless the policy is set.

llvm/include/llvm/CodeGen/MachineScheduler.h

struct MachineSchedPolicy {
   // is true, the scheduler runs in both directions and converges.
   bool OnlyTopDown = false;
   bool OnlyBottomUp = false;
 
   // Disable heuristic that tries to fetch nodes from long dependency chains
   // first.
   bool DisableLatencyHeuristic = false;
 
+  // Compute DFSResult for use in scheduling heuristics.
+  bool ComputeDFSResult = false;
+
   MachineSchedPolicy() = default;
 };
 
 /// MachineSchedStrategy - Interface to the scheduling algorithm used by
 /// ScheduleDAGMI.
 ///
 /// Initialization sequence:
 ///   initPolicy -> shouldTrackPressure -> initialize(DAG) -> registerRoots

llvm/lib/CodeGen/MachineScheduler.cpp

 
 void GenericScheduler::initialize(ScheduleDAGMI *dag) {
   assert(dag->hasVRegLiveness() &&
          "(PreRA)GenericScheduler needs vreg liveness");
   DAG = static_cast<ScheduleDAGMILive*>(dag);
   SchedModel = DAG->getSchedModel();
   TRI = DAG->TRI;
 
+  if (RegionPolicy.ComputeDFSResult)
+    DAG->computeDFSResult();
+
   Rem.init(DAG, SchedModel);
   Top.init(DAG, SchedModel, &Rem);
   Bot.init(DAG, SchedModel, &Rem);
 
   // Initialize resource counts.
 
   // Initialize the HazardRecognizers. If itineraries don't exist, are empty, or
   // are disabled, then these HazardRecs will be disabled.