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

[erifan]

On Wed, 20 Aug 2025 11:27:59 GMT, Andrew Haley <aph 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 I have I basically copied the algorithm description to the vector expand implementation. Since there are already two examples in the implementation functions, I didn't copy this example over. Could you review it again? Thank you.

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PR Comment: https://git.openjdk.org/jdk/pull/26740#issuecomment-3209252469