RFR: fix loop unswitching with -XX:-ShenandoahWriteBarrierMemBar

[Roland Westrelin]

> With the patch, -XX:-ShenandoahWriteBarrierMemBar does not crash 
> anymore, but it's significantly slower than with membar... like 75% 
> slower. Which seems illogical.
>
> tried with:
> -XX:+UseShenandoahGC -XX:+UnlockDiagnosticVMOptions 
> -XX:+UnlockExperimentalVMOptions -Xms4g -Xmx4g 
> -XX:ShenandoahGCHeuristics=traversal -XX:ShenandoahFreeThreshold=17 
> -XX:-ShenandoahWriteBarrierMemBar
>
> on specjvm compiler.

I don't see that:

-ShenandoahWriteBarrierMemBar
Compiler.compiler  thrpt   30  378.604 ± 18.569  ops/min

+ShenandoahWriteBarrierMemBar
Compiler.compiler  thrpt   30  398.564 ± 23.585  ops/min

current repo with a product build.

Roland.