RFR: 8283008: KRegister documentation out of date

[Jie Fu]

On Fri, 11 Mar 2022 08:17:07 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

> Hi all,
> 
>   can I have reviews for this cleanup of `KRegister(Impl)` documentation, currently being a mix of copy&paste from `XMMRegister` and obsolete MMX support.
> 
> Testing: local compilation
> 
> Thanks,
>   Thomas

Please also update the copyright year.

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PR: https://git.openjdk.java.net/jdk/pull/7784