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

[Richard Reingruber]

On Tue, 21 Feb 2023 03:58:50 GMT, Martin Doerr <mdoerr at openjdk.org> wrote:

> 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`.)

Looks good.

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Marked as reviewed by rrich (Reviewer).

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