RFR: fix aarch64 membar elision with shenandoah

[Roman Kennke]

Am 16.07.2018 um 12:09 schrieb Roland Westrelin:
> 
>> Have you posted a fix for the failing test? Or do you want us to take
>> care of it after you pushed?
> 
> I haven't. Not sure how you want this fixed (fix the test? disallow
> combination of flags?) so it seems best if you handle it.


OK, then please push, I'll fix the test then.

Thank you!
Roman