RFR: 8335604: Serial: Inline Generation::contiguous_available

[Albert Mingkun Yang]

Trivial inlining a method.

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Commit messages:
 - s1-inline

Changes: https://git.openjdk.org/jdk/pull/20000/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20000&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335604
  Stats: 18 lines in 5 files changed: 0 ins; 17 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/20000.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20000/head:pull/20000

PR: https://git.openjdk.org/jdk/pull/20000