RFR: 8266440: Shenandoah: TestReferenceShortcutCycle.java test failed on AArch64

[Zhengyu Gu]

On Fri, 7 May 2021 12:33:35 GMT, Zhengyu Gu <zgu at openjdk.org> wrote:

>> We don't need a register. It's just a TBNZ and a TBZ, isn't it?
>
> Unfortunately, tmp1 == noreg here.

so, this is better alternative?
    `Label lrb;`
    `__ tbnz(rscratch2, ShenandoahHeap::HAS_FORWARDED_BITPOS, lrb);`
    `__ tbnz(rscratch2, ShenandoahHeap::WEAK_ROOTS_BITPOS, lrb);`
    `__ b(heap_stable);`
    `__ bind(lrb);`

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PR: https://git.openjdk.java.net/jdk/pull/3902