Implement local check elimination on basic blocks.

src/hydrogen-instructions.h

Index: src/hydrogen-instructions.h
diff --git a/src/hydrogen-instructions.h b/src/hydrogen-instructions.h
index 8cb2f591742aebec491d57d4ab9305b51cc1eeb4..022a7a143118d4de304f987989e2f4e176db181f 100644
--- a/src/hydrogen-instructions.h
+++ b/src/hydrogen-instructions.h
@@ -3482,6 +3482,13 @@ class HConstant V8_FINAL : public HTemplateInstruction<0> {
     return Unique<Object>(raw_address, handle_);
   }
 
+  template<typename T>
+  Unique<T> GetCastedUnique() const {
+    // TODO(titzer): store a Unique<HeapObject> inside the HConstant.
+    Address raw_address = reinterpret_cast<Address>(unique_id_.Hashcode());
+    return Unique<T>(raw_address, Handle<T>::cast(handle_));
+  }
+
 #ifdef DEBUG
   virtual void Verify() V8_OVERRIDE { }
 #endif