[jdk21u] RFR: 8320363: ppc64 TypeEntries::type_unknown logic looks wrong, missed optimization opportunity

[Martin Doerr]

Clean backport of [JDK-8320363](https://bugs.openjdk.org/browse/JDK-8320363).

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Commit messages:
 - Backport 6aa197667ad05bd93adf3afc7b06adbfb2b18a22

Changes: https://git.openjdk.org/jdk21u/pull/403/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u&pr=403&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320363
  Stats: 2 lines in 2 files changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk21u/pull/403.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u.git pull/403/head:pull/403

PR: https://git.openjdk.org/jdk21u/pull/403