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

src/compiler/typer.cc

Index: src/compiler/typer.cc
diff --git a/src/compiler/typer.cc b/src/compiler/typer.cc
index a4e3c64911f08fd4b58c73b3dcd7f31cb9964feb..aececc004da247fc91c5c227b164dec2d7f38832 100644
--- a/src/compiler/typer.cc
+++ b/src/compiler/typer.cc
@@ -719,6 +719,10 @@ Type* Typer::Visitor::TypeLoopExitEffect(Node* node) {
   return nullptr;
 }
 
+Type* Typer::Visitor::TypeEnsureWritableFastElements(Node* node) {
+  return Operand(node, 1);
+}
+
 Type* Typer::Visitor::TypeTransitionElementsKind(Node* node) {
   UNREACHABLE();
   return nullptr;