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

[Martin Doerr]

We should use `constexpr` more often in order to make it better usable. (I got compile errors when trying to use the `encoding()` in another `constexpr`.)

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Commit messages:
 - 8302907: [PPC64] Use more constexpr in class Register

Changes: https://git.openjdk.org/jdk/pull/12681/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12681&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/jdk/pull/12681.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12681/head:pull/12681

PR: https://git.openjdk.org/jdk/pull/12681