RFR: 8328278: Serial: Compute tenuring threshold before GC

[Guoxiong Li]

Hi all,

This patch places the SerialGC code that computes tenuring threshold before per GC instead of after per GC, so that the method `AgeTable::print_on` prints the right tenuring threshold. Just like the current G1 implementation.

Thanks for taking the time to review.

Best Regards,
-- Guoxiong

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Commit messages:
 - JDK-8328278

Changes: https://git.openjdk.org/jdk/pull/18329/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18329&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8328278
  Stats: 5 lines in 1 file changed: 1 ins; 2 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/18329.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18329/head:pull/18329

PR: https://git.openjdk.org/jdk/pull/18329