RFR: 8260106: Shenandoah: refactor reference updating closures and related code [v6]

[Zhengyu Gu]

On Mon, 25 Jan 2021 15:58:00 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

>> We have a block in `ShenandoahHeap::maybe_update_with_forwarded` that is irrelevant after JDK-8231086. Additionally, "resolve and update" paths are really only used by STW GCs, and thus do not require atomic updates. This leads to considerable simplifications in the code, and improves performance on the common paths (especially in fastdebug builds that drop many irrelevant asserts).
>> 
>> Additional testing:
>>  - [x] `hotspot_gc_shenandoah`
>>  - [x] `tier1` with Shenandoah
>>  - [x] `tier2` with Shenandoah
>
> Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Add const

Still good.

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Marked as reviewed by zgu (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/2166