RFR: one more missing UseShenandoahGC check in 8u

[Roland Westrelin]

> Okay. Seems cleaner just to split the paths?
>
>  Node zlen_arg = NULL;
>  if (UseShenandoahGC) {
>    Node *cast = new (C) CastPPNode(z, TypePtr::NOTNULL);
>    cast->init_req(0, control());
>    _gvn.set_type(cast, cast->bottom_type());
>    C->record_for_igvn(cast);
>    zlen_arg = load_array_length(cast);
>  } else {
>    zlen_arg = load_array_length(z);
>  }

Ok. Should I wait for more reviews or can I push it?

Roland.