[turbofan] Revive the VectorSlotPair and also put feedback on JSCallFunction.

src/handles.h

Index: src/handles.h
diff --git a/src/handles.h b/src/handles.h
index eb57f0e26089383c9d7fe9f57a8cba5f41a111e2..7a296fe68a16acb2f1d05a076f1c84614bf82970 100644
--- a/src/handles.h
+++ b/src/handles.h
@@ -66,14 +66,32 @@ class MaybeHandle {
 
   bool is_null() const { return location_ == NULL; }
 
+  template <typename S>
+  bool operator==(MaybeHandle<S> that) const {
+    return this->location_ == that.location_;
+  }
+  template <typename S>
+  bool operator!=(MaybeHandle<S> that) const {
+    return !(*this == that);
+  }
+
+
  protected:
   T** location_;
 
   // MaybeHandles of different classes are allowed to access each
   // other's location_.
   template<class S> friend class MaybeHandle;
+  template <typename S>
+  friend size_t hash_value(MaybeHandle<S>);
 };
 
+template <typename S>
+inline size_t hash_value(MaybeHandle<S> maybe_handle) {
+  return bit_cast<size_t>(maybe_handle.location_);
+}
+
+
 // ----------------------------------------------------------------------------
 // A Handle provides a reference to an object that survives relocation by
 // the garbage collector.