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

[Albert Mingkun Yang]

> 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

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Changes:
  - all: https://git.openjdk.org/jdk/pull/12471/files
  - new: https://git.openjdk.org/jdk/pull/12471/files/c51fffa9..323d19f0

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=12471&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=12471&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/12471.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12471/head:pull/12471

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