Try convert polymorphic stores into a single monomorphic store.

src/property-details.h

Index: src/property-details.h
diff --git a/src/property-details.h b/src/property-details.h
index 669b05dca051d8feefa413410f2afb666b5c3979..b0d10e1270aaaceca63430166ef5524e90043598 100644
--- a/src/property-details.h
+++ b/src/property-details.h
@@ -110,6 +110,10 @@ class Representation {
         (!IsDouble() && !other.IsDouble());
   }
 
+  bool IsCompatibleForStore(const Representation& other) const {
+    return Equals(other);
+  }
+
   bool is_more_general_than(const Representation& other) const {
     ASSERT(kind_ != kExternal);
     ASSERT(other.kind_ != kExternal);