RFR: 8324856: Serial: Move Generation::is_in to DefNewGeneration

[Albert Mingkun Yang]

Trivial moving code from superclass to subclass.

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Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/17616/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17616&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8324856
  Stats: 36 lines in 4 files changed: 9 ins; 27 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/17616.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17616/head:pull/17616

PR: https://git.openjdk.org/jdk/pull/17616