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

src/compiler/js-graph.cc

Index: src/compiler/js-graph.cc
diff --git a/src/compiler/js-graph.cc b/src/compiler/js-graph.cc
index ea3ec5e6dd1ed4be43a2961a4f9b23c9d1dff96f..d8a8af412a9cbcad453d91a21c0648473809d557 100644
--- a/src/compiler/js-graph.cc
+++ b/src/compiler/js-graph.cc
@@ -52,6 +52,11 @@ Node* JSGraph::EmptyStringConstant() {
   return CACHED(kEmptyStringConstant, HeapConstant(factory()->empty_string()));
 }
 
+Node* JSGraph::FixedArrayMapConstant() {
+  return CACHED(kFixedArrayMapConstant,
+                HeapConstant(factory()->fixed_array_map()));
+}
+
 Node* JSGraph::HeapNumberMapConstant() {
   return CACHED(kHeapNumberMapConstant,
                 HeapConstant(factory()->heap_number_map()));