[turbofan] Add support for copy-on-write element stores.

src/compiler/js-graph.h

Index: src/compiler/js-graph.h
diff --git a/src/compiler/js-graph.h b/src/compiler/js-graph.h
index a9e73575426a9fecd6d367c79503d92885d17860..f515389d7fbb45cb1521dcb632384fc091a6d7bf 100644
--- a/src/compiler/js-graph.h
+++ b/src/compiler/js-graph.h
@@ -46,6 +46,7 @@ class JSGraph : public ZoneObject {
   Node* EmptyFixedArrayConstant();
   Node* EmptyLiteralsArrayConstant();
   Node* EmptyStringConstant();
+  Node* FixedArrayMapConstant();
   Node* HeapNumberMapConstant();
   Node* OptimizedOutConstant();
   Node* StaleRegisterConstant();
@@ -156,6 +157,7 @@ class JSGraph : public ZoneObject {
     kEmptyFixedArrayConstant,
     kEmptyLiteralsArrayConstant,
     kEmptyStringConstant,
+    kFixedArrayMapConstant,
     kHeapNumberMapConstant,
     kOptimizedOutConstant,
     kStaleRegisterConstant,