Fix SubMov copying

inputs/passing/copyCounting/subscriptAssignment.ts

@@ -1,13 +1,15 @@
 //! test_output(2)
-// Should be: 1
 
 /// <reference path="../../../concept-code/vs.d.ts" />
 
 export default function main() {
   const x = Debug.makeCopyCounter("x");
 
-  let obj: Record<string, unknown> = { x }; // Single copy occurs here
-  obj.y = "y"; // Shouldn't copy, but does
+  let obj: Record<string, unknown> = { x }; // First copy
+  obj.y = "y"; // No extra copy
+
+  let arr: unknown[] = [x]; // Second copy
+  arr[1] = "y"; // No extra copy
 
   return x.count;
 }