[RegAllocGreedy] Provide a subtarget hook to disable the local reassignment

heuristic.
By default, no functionality change.
This is a follow-up of r212099.

This hook provides a finer grain to control the optimization.

<rdar://problem/17444599>


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212204 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Target/TargetSubtargetInfo.h

@@ -100,6 +100,12 @@ public:
                                      AntiDepBreakMode& Mode,
                                      RegClassVector& CriticalPathRCs) const;
 
+  /// \brief True if the subtarget should run the local reassignment
+  /// heuristic of the register allocator.
+  /// This heuristic may be compile time intensive, \p OptLevel provides
+  /// a finer grain to tune the register allocator.
+  virtual bool enableRALocalReassignment(CodeGenOpt::Level OptLevel) const;
+
   /// \brief Enable use of alias analysis during code generation (during MI
   /// scheduling, DAGCombine, etc.).
   virtual bool useAA() const;