RFR: 8302068: Serial: Refactor oop closures used in Young GC [v2]

[Ivan Walulya]

On Fri, 10 Feb 2023 15:05:21 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 with a new target base due to a merge or a rebase. The pull request now contains three commits:
> 
>  - merge
>  - Merge branch 'master' into gen-oop
>  - gen-oop

Lgtm!

Minor:
Few uses of NULL instead of nullptr.

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Marked as reviewed by iwalulya (Reviewer).

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