Use weak cell to embed map in CompareNil IC.

src/hydrogen-instructions.h

Index: src/hydrogen-instructions.h
diff --git a/src/hydrogen-instructions.h b/src/hydrogen-instructions.h
index b933f10ba80f266d67f3ead95ca21a42ee43f216..1fc49cc85f57098502ffdcb09e53a4f97220e480 100644
--- a/src/hydrogen-instructions.h
+++ b/src/hydrogen-instructions.h
@@ -6251,6 +6251,10 @@ class HObjectAccess FINAL {
     return HObjectAccess(kInobject, Cell::kValueOffset);
   }
 
+  static HObjectAccess ForWeakCellValue() {
+    return HObjectAccess(kInobject, WeakCell::kValueOffset);
+  }
+
   static HObjectAccess ForAllocationMementoSite() {
     return HObjectAccess(kInobject, AllocationMemento::kAllocationSiteOffset);
   }