[jdk17u-dev] RFR: 8302907: [PPC64] Use more constexpr in class Register

[Martin Doerr]

Clean backport of [JDK-8302907](https://bugs.openjdk.org/browse/JDK-8302907).

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Commit messages:
 - Backport 180b94c73e9ad17d57650d4c985d4104289052a9

Changes: https://git.openjdk.org/jdk17u-dev/pull/2461/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=2461&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302907
  Stats: 17 lines in 1 file changed: 0 ins; 0 del; 17 mod
  Patch: https://git.openjdk.org/jdk17u-dev/pull/2461.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev.git pull/2461/head:pull/2461

PR: https://git.openjdk.org/jdk17u-dev/pull/2461