RFR: 8260106: Shenandoah: simplify maybe_update_with_forwarded and related code [v2]

[Roman Kennke]

On Thu, 21 Jan 2021 08:06:10 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`
>>  - [ ] `tier1` with Shenandoah
>>  - [ ] `tier2` with Shenandoah
>
> Aleksey Shipilev has updated the pull request incrementally with two additional commits since the last revision:
> 
>  - Rename maybe to atomic
>  - Touch up comments

Looks good! Thanks!

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Marked as reviewed by rkennke (Reviewer).

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