misched: Allow subtargets to enable misched and dependent options.

This allows me to begin enabling (or backing out) misched by default
for one subtarget at a time. To run misched we typically want to:
- Disable SelectionDAG scheduling (use the source order scheduler)
- Enable more aggressive coalescing (until we decide to always run the coalescer this way)
- Enable MachineScheduler pass itself.

Disabling PostRA sched may follow for some subtargets.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@167826 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Target/TargetSubtargetInfo.h

@@ -54,6 +54,13 @@ public:
     return 0;
   }
 
+  /// \brief True if the subtarget should run MachineScheduler after aggressive
+  /// coalescing.
+  ///
+  /// This currently replaces the SelectionDAG scheduler with the "source" order
+  /// scheduler. It does not yet disable the postRA scheduler.
+  virtual bool enableMachineScheduler() const;
+
   // enablePostRAScheduler - If the target can benefit from post-regalloc
   // scheduling and the specified optimization level meets the requirement
   // return true to enable post-register-allocation scheduling. In