Fix using monomorphic store instruction for polymorphic stores.

src/hydrogen.cc

Index: src/hydrogen.cc
diff --git a/src/hydrogen.cc b/src/hydrogen.cc
index 9a2072ec9f054cfc311f0265c41fb07ead0f124b..396e98a7f931cd824e516d3faef20a94c1ec97e0 100644
--- a/src/hydrogen.cc
+++ b/src/hydrogen.cc
@@ -6408,7 +6408,8 @@ bool HOptimizedGraphBuilder::TryStorePolymorphicAsMonomorphic(
   AddInstruction(HCheckMaps::New(object, types, zone()));
   HInstruction* store;
   CHECK_ALIVE_OR_RETURN(
-      store = BuildStoreNamedField(object, name, value, types->at(0), &lookup),
+      store = BuildStoreNamedField(
+          object, name, value, types->at(count - 1), &lookup),
       true);
   Push(value);
   store->set_position(expr->position());