[jdk21u-dev] RFR: 8335252: Reduce size of j.u.Formatter.Conversion#isValid

[Paul Hohensee]

Clean backport to fix a regression resulting from [JDK-8263038](https://bugs.openjdk.org/browse/JDK-8263038).

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Commit messages:
 - Backport 5d866bf17d96bd0f0e4545d7eee5912eda2e3a94

Changes: https://git.openjdk.org/jdk21u-dev/pull/1827/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=1827&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335252
  Stats: 3 lines in 1 file changed: 0 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/1827.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/1827/head:pull/1827

PR: https://git.openjdk.org/jdk21u-dev/pull/1827