RFR: 8299323: Allow extended registers for cmpw

[Erik Österlund]

On Tue, 3 Jan 2023 23:38:10 GMT, Sandhya Viswanathan <sviswanathan 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.
>
> LGTM

Thanks for the review @sviswa7!

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