RFR: fix loop unswitching with -XX:-ShenandoahWriteBarrierMemBar

[Roland Westrelin]

>>-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.
>
> Yes. But without membars, I would expect it to be faster or equal compared to with membars. But not slower... Or am I missing something?

Those membars are noop on x86 so perf should be the same. Given how much
compiler numbers vary, it's hard to tell there's a regression from the
numbers above, AFAICT.

Roland.