RFR: Fix barrier set cast for aarch64

[William Kemper]

On Thu, 29 Apr 2021 23:35:26 GMT, Zhengyu Gu <zgu at openjdk.org> wrote:

>> Simple fix for aarch64 (`ShenandoahBarrierSet` no longer extends from `CardTableBarrierSet`).
>
> src/hotspot/cpu/aarch64/gc/shenandoah/shenandoahBarrierSetAssembler_aarch64.cpp line 630:
> 
>> 628: 
>> 629:   BarrierSet* bs = BarrierSet::barrier_set();
>> 630:   ShenandoahBarrierSet* ctbs = barrier_set_cast<ShenandoahBarrierSet>(bs);
> 
> Can be simplified to ShenandoahBarrierSet::barrier_set(). Otherwise, looks good.

Fixed.

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PR: https://git.openjdk.java.net/shenandoah/pull/33