RFR: 8303968: Serial: Use more precise liveness info in Young GC reference processing [v2]

[Albert Mingkun Yang]

> 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

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 two commits:

 - review
 - s1-ref-is-alive

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Changes: https://git.openjdk.org/jdk/pull/12975/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12975&range=01
  Stats: 32 lines in 2 files changed: 15 ins; 7 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/12975.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/12975/head:pull/12975

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