RFR: 8299323: Allow extended registers for cmpw

[Vladimir Kozlov]

On Fri, 23 Dec 2022 12:41:03 GMT, Erik Österlund <eosterlund 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.

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Marked as reviewed by kvn (Reviewer).

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