Bug: C2 crash on Derby with -XX:-ShenandoahNoBarriersForConst

[Roland Westrelin]

> Even after the recent Roland's fix, Derby fails with ShNoBarriersForConst:
>
> $ java -jar target/benchmarks.jar Derby --jvmArgs "-XX:+UseShenandoahGC -Xmx4g
> -Xms4g -XX:+UnlockExperimentalVMOptions -XX:-ShenandoahNoBarriersForConst" -f 1 -t 8
>
> http://cr.openjdk.java.net/~shade/shenandoah/crashes/derby-noconst/hs_err_pid28482.log
>
> http://cr.openjdk.java.net/~shade/shenandoah/crashes/derby-noconst/replay_pid28482.log

FYI, this one is still not fixed. I pushed some fixes today but they are
unrelated. I'm working on the Derby crash now.

Roland.