RFR: 8302068: Serial: Refactor oop closures used in Young GC [v3]

[Albert Mingkun Yang]

On Mon, 20 Feb 2023 12:43:04 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> The diff can be a bit hard to follow; it's best to view new code on its own.
>> 
>> Test: tier1-5
>
> Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:
> 
>   nullptr

Thanks for the review.

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PR: https://git.openjdk.org/jdk/pull/12471