misched: implemented a framework for top-down or bottom-up scheduling.

New flags: -misched-topdown, -misched-bottomup. They can be used with
the default scheduler or with -misched=shuffle. Without either
topdown/bottomup flag -misched=shuffle now alternates scheduling
direction.

LiveIntervals update is unimplemented with bottom-up scheduling, so
only -misched-topdown currently works.

Capped the ScheduleDAG hierarchy with a concrete ScheduleDAGMI class.
ScheduleDAGMI is aware of the top and bottom of the unscheduled zone
within the current region. Scheduling policy can be plugged into
the ScheduleDAGMI driver by implementing MachineSchedStrategy.
ConvergingScheduler is now the default scheduling algorithm.
It exercises the new driver but still does no reordering.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@152700 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/ScheduleDAG.h

@@ -410,6 +410,13 @@ namespace llvm {
       return false;
     }
 
+    bool isTopReady() const {
+      return NumPredsLeft == 0;
+    }
+    bool isBottomReady() const {
+      return NumSuccsLeft == 0;
+    }
+
     void dump(const ScheduleDAG *G) const;
     void dumpAll(const ScheduleDAG *G) const;
     void print(raw_ostream &O, const ScheduleDAG *G) const;