RFR: 8266813: Use shorter instruction sequence for checking if marking in progress

[Aleksey Shipilev]

On Mon, 10 May 2021 13:21:44 GMT, Zhengyu Gu <zgu at openjdk.org> wrote:

> ShenandoahBarrierSetAssembler::generate_c1_pre_barrier_runtime_stub() on aarch64, we can replace ld, tst, br instruction sequence with a single tbz.
> 
> Test:
> - [x] hotspot_gc_shenandoah on Linux aarch64

This looks fine to me.

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Marked as reviewed by shade (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/3948