RFR: 8348868: AArch64: Add backend support for SelectFromTwoVector [v3]

[Bhavana Kilambi]

On Wed, 18 Jun 2025 08:30:36 GMT, Xiaohong Gong <xgong at openjdk.org> wrote:

> To check whether it is an issue, you can use 64bits and 128bits as an example. And change to use SVE2's tbl for this op with 64bits.

Yes, I tried that and it does give incorrect results. 
It does an ldr into `d` regs but the tbl is on `z` regs. The values in the `index` are not a problem. They will be generated according to the vector length (in this case 64-bit) but since the `tbl` does a full register lookup (and not partial), it can return the higher bits of the `z` register as well.

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PR Review Comment: https://git.openjdk.org/jdk/pull/23570#discussion_r2154003610