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

[Aleksey Shipilev]

On Thu, 21 Jan 2021 08:59:07 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

>> 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!

I updated the patch a bit after `RESOLVE` mode removal. I think `NO_UPDATE` and `STW_UPDATE` reads better. This also highlights that STW closures are really doing non-CAS updates.

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