With StressIGVN, we shuffle_worklist at the beginning of PhaseIterGVN::optimize and then linearly work down the worklist. This means that nodes that get put on the worklist during IGVN are not randomized.

We should instead do it like in PhaseCCP::analyze, where we always ask for the next node via PhaseCCP::fetch_next_node, which picks a random next node from the worklist if StressCCP is enabled.