LibJS: Optimize array destructuring assignment for builtin iterators

...by avoiding `{ value, done }` iterator result value allocation. This
change applies the same otimization 81b6a11 added for `for..in` and
`for..of`.

Makes following micro benchmark go 22% faster on my computer:
```js
function f() {
    const arr = [];
    for (let i = 0; i < 10_000_000; i++) {
        arr.push([i]);
    }
    let sum = 0;
    for (let [i] of arr) {
        sum += i;
    }
}

f();
```

Libraries/LibJS/Bytecode/Instruction.h

@@ -51,7 +51,6 @@
     O(EnterObjectEnvironment)          \
     O(EnterUnwindContext)              \
     O(Exp)                             \
-    O(ForOfNext)                       \
     O(GetById)                         \
     O(GetByIdWithThis)                 \
     O(GetByValue)                      \
@@ -81,6 +80,7 @@
     O(InstanceOf)                      \
     O(IteratorClose)                   \
     O(IteratorNext)                    \
+    O(IteratorNextUnpack)              \
     O(IteratorToArray)                 \
     O(Jump)                            \
     O(JumpFalse)                       \