RFR: fix aarch64 membar elision with shenandoah

[Aleksey Shipilev]

On 07/13/2018 04:40 PM, Roland Westrelin wrote:
> 
>> The test is used to verify that we do not emit barriers when particular types of barriers are
>> disabled: there are asserts all over runtime, C1 and C2 code to test these. Over time, it started to
>> be the test to make sure that arguments/heuristics code verifies and drops the incompatible
>> combination of flags.
> 
> Can we simply skip barrier elision in case of a strange combination of
> options? It's a pain to fix that code.

Yup. See how shenandoahArguments.cpp does the same for ShenandoahVerifyOptoBarriers.

-Aleksey