RFR: 8303968: Serial: Use more precise liveness info in Young GC reference processing

[Thomas Schatzl]

On Fri, 10 Mar 2023 13:44:18 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

> Simple refactoring around Young-GC keep-alive-closure.
> 
> (I went for the static-local-var approach, as other solutions that I am aware of would expose the is-alive-closure in the header.)
> 
> Test: tier1-3

Lgtm.

I would still prefer to move the static to the class into the header file instead of hiding it where it is currently located. All other (STW-)GCs have their `IsAliveClosure` declared next to the reference processor static variable too.

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Marked as reviewed by tschatzl (Reviewer).

PR: https://git.openjdk.org/jdk/pull/12975