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

src/compiler/simplified-lowering.cc

Index: src/compiler/simplified-lowering.cc
diff --git a/src/compiler/simplified-lowering.cc b/src/compiler/simplified-lowering.cc
index 297d18131024f80d6392dd683fb99a9842487148..c04e6217c978fdf28c9ccb857ebe5dbdafda0c7a 100644
--- a/src/compiler/simplified-lowering.cc
+++ b/src/compiler/simplified-lowering.cc
@@ -2375,6 +2375,9 @@ class RepresentationSelector {
         VisitInputs(node);
         return SetOutput(node, MachineRepresentation::kNone);
       }
+      case IrOpcode::kEnsureWritableFastElements:
+        return VisitBinop(node, UseInfo::AnyTagged(),
+                          MachineRepresentation::kTagged);
 
       //------------------------------------------------------------------
       // Machine-level operators.