RFR: Fix barrier set cast for aarch64

[Zhengyu Gu]

On Thu, 29 Apr 2021 22:00:03 GMT, William Kemper <wkemper at openjdk.org> wrote:

> Simple fix for aarch64 (`ShenandoahBarrierSet` no longer extends from `CardTableBarrierSet`).

Marked as reviewed by zgu (Reviewer).

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.

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