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

src/compiler/simplified-operator.cc

Index: src/compiler/simplified-operator.cc
diff --git a/src/compiler/simplified-operator.cc b/src/compiler/simplified-operator.cc
index e7374246961e3011569ffb646cd90b92f5fb497c..989aa3613e0337bdd9c72b47313bf1d2c624bcfa 100644
--- a/src/compiler/simplified-operator.cc
+++ b/src/compiler/simplified-operator.cc
@@ -503,6 +503,16 @@ struct SimplifiedOperatorGlobalCache final {
   AllocateOperator<NOT_TENURED> kAllocateNotTenuredOperator;
   AllocateOperator<TENURED> kAllocateTenuredOperator;
 
+  struct EnsureWritableFastElementsOperator final : public Operator {
+    EnsureWritableFastElementsOperator()
+        : Operator(                                     // --
+              IrOpcode::kEnsureWritableFastElements,    // opcode
+              Operator::kNoDeopt | Operator::kNoThrow,  // flags
+              "EnsureWritableFastElements",             // name
+              2, 1, 1, 1, 1, 0) {}                      // counts
+  };
+  EnsureWritableFastElementsOperator kEnsureWritableFastElements;
+
 #define BUFFER_ACCESS(Type, type, TYPE, ctype, size)                          \
   struct LoadBuffer##Type##Operator final : public Operator1<BufferAccess> {  \
     LoadBuffer##Type##Operator()                                              \
@@ -618,6 +628,10 @@ const Operator* SimplifiedOperatorBuilder::ReferenceEqual(Type* type) {
                                "ReferenceEqual", 2, 0, 0, 1, 0, 0);
 }
 
+const Operator* SimplifiedOperatorBuilder::EnsureWritableFastElements() {
+  return &cache_.kEnsureWritableFastElements;
+}
+
 const Operator* SimplifiedOperatorBuilder::TransitionElementsKind(
     ElementsTransition transition) {
   return new (zone()) Operator1<ElementsTransition>(  // --