RFR: 8366076: arm32: Fix register allocation for vector instructions [v2]

[Max Verevkin]

On Tue, 18 Nov 2025 05:23:14 GMT, Dean Long <dlong at openjdk.org> wrote:

>> I am not 100% sure if they are completely equivalent and `dflt_low_reg` could be used instead of defining a new class. I figured I should introduce a new class similar to how `sflt_reg` and `dflt_low_reg` are similar yet distinct.
>
> A reg_class just produces a RegMask, so there is no need to give identical masks different names.  Aliasing can still be done.  See `actual_dflt_reg` for example.

Sorry for such a long delay. Should be addressed now :)

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PR Review Comment: https://git.openjdk.org/jdk/pull/27071#discussion_r2571965840