RFR: 8363989: AArch64: Add missing backend support of VectorAPI expand operation

[Emanuel Peter]

On Wed, 3 Sep 2025 10:11:38 GMT, erifan <duke at openjdk.org> wrote:

>> The algorithm description here is great. Please paste all of it from "Since there are" to "but with different instructions where appropriate." into this PR, before the vector expand implementation.
>
> @theRealAph @e1iu @XiaohongGong @fg1417 @shqking, could you help take a look at this PR, thanks~

@erifan Feel free to ping me again if I should re-review. I'm going on vacation in a week, so I'll be unresponsive for a while (feel free to contact other reviewers, especially for additional testing).

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PR Comment: https://git.openjdk.org/jdk/pull/26740#issuecomment-3269684547