[turbofan] Treat PREMONOMORPHIC like UNINITIALIZED.

src/type-feedback-vector.h

Index: src/type-feedback-vector.h
diff --git a/src/type-feedback-vector.h b/src/type-feedback-vector.h
index 5355ee71884a1096c0c185f0b5c4ac097684d2e3..30d6e83c3a8e2a0b288f0ad347c70b870525a699 100644
--- a/src/type-feedback-vector.h
+++ b/src/type-feedback-vector.h
@@ -400,6 +400,9 @@ class FeedbackNexus {
 
   InlineCacheState ic_state() const { return StateFromFeedback(); }
   bool IsUninitialized() const { return StateFromFeedback() == UNINITIALIZED; }
+  bool IsPremonomorphic() const {
+    return StateFromFeedback() == PREMONOMORPHIC;
+  }
   Map* FindFirstMap() const {
     MapHandleList maps;
     ExtractMaps(&maps);