MachineCSE: Add a target query for the LookAheadLimit heurisitic

This is used to determine whether or not to CSE physical register
defs.

Differential Revision: http://reviews.llvm.org/D9472

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@236923 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Target/TargetInstrInfo.h

@@ -1235,6 +1235,14 @@ public:
     return false;
   }
 
+  /// \brief Return the value to use for the MachineCSE's LookAheadLimit,
+  /// which is a heuristic used for CSE'ing phys reg defs.
+  virtual unsigned getMachineCSELookAheadLimit () const {
+    // The default lookahead is small to prevent unprofitable quadratic
+    // behavior.
+    return 5;
+  }
+
 private:
   int CallFrameSetupOpcode, CallFrameDestroyOpcode;
 };