RFR: 8299323: Allow extended registers for cmpw

[Erik Österlund]

On Mon, 16 Jan 2023 23:15:52 GMT, Vladimir Kozlov <kvn at openjdk.org> wrote:

>> The current instruction encoder for cmpw(Address, int16_t) on x64 does not allow rex extended registers. Generational ZGC needs to use this for arbitrary registers. Let's add support for it instead of asserting the input Address uses a subset of registers.
>
> Looks reasonable.
> Did you verify that the code works when extended registers are used? It does match how we encode other 16bit instructions.

Thanks for the review, @vnkozlov! I did indeed check that the code generation is accurate for extended registers.

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PR: https://git.openjdk.org/jdk/pull/11776