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

src/compiler/simplified-operator.h

Index: src/compiler/simplified-operator.h
diff --git a/src/compiler/simplified-operator.h b/src/compiler/simplified-operator.h
index d6b0af8b7755247b0cdd2586e3b0ed75aed1bf5a..fac19f0882e6b02d2c52ec9a136577f6e5ebf9bd 100644
--- a/src/compiler/simplified-operator.h
+++ b/src/compiler/simplified-operator.h
@@ -148,6 +148,8 @@ enum class ElementsTransition : uint8_t {
   kSlowTransition   // full transition, round-trip to the runtime.
 };
 
+size_t hash_value(ElementsTransition);
+
 std::ostream& operator<<(std::ostream&, ElementsTransition);
 
 ElementsTransition ElementsTransitionOf(const Operator* op) WARN_UNUSED_RESULT;
@@ -311,6 +313,9 @@ class SimplifiedOperatorBuilder final : public ZoneObject {
   const Operator* ObjectIsString();
   const Operator* ObjectIsUndetectable();
 
+  // ensure-writable-fast-elements object, elements
+  const Operator* EnsureWritableFastElements();
+
   // transition-elements-kind object, from-map, to-map
   const Operator* TransitionElementsKind(ElementsTransition transition);