LibJS: Stop converting between Object <-> IteratorRecord all the time

This patch makes IteratorRecord an Object. Although it's not exposed to
author code, this does allow us to store it in a VM register.

Now that we can store it in a VM register, we don't need to convert it
back and forth between IteratorRecord and Object when accessing it from
bytecode.

The big win here is avoiding 3 [[Get]] accesses on every iteration step
of for..of loops. There are also a bunch of smaller efficiencies gained.

20% speed-up on this microbenchmark:

    function go(a) {
        for (const p of a) {
        }
    }
    const a = [];
    a.length = 1_000_000;
    go(a);

Userland/Libraries/LibJS/JIT/Compiler.h

@@ -114,6 +114,8 @@ private:
         O(BlockDeclarationInstantiation, block_declaration_instantiation)        \
         O(SuperCallWithArgumentArray, super_call_with_argument_array)            \
         O(GetIterator, get_iterator)                                             \
+        O(GetObjectFromIteratorRecord, get_object_from_iterator_record)          \
+        O(GetNextMethodFromIteratorRecord, get_next_method_from_iterator_record) \
         O(IteratorNext, iterator_next)                                           \
         O(IteratorResultDone, iterator_result_done)                              \
         O(ThrowIfNotObject, throw_if_not_object)                                 \