RFR: 8355239: RISC-V: Do not support subword scatter store

[Fei Yang]

On Tue, 22 Apr 2025 02:47:02 GMT, Fei Yang <fyang at openjdk.org> wrote:

> Hi, please consider this small enhancement change.
> 
> Currently, only word and double-word gather load and scatter store are supported on riscv.
> Both subword gather load and scatter store are not supported due to constraint of riscv vector extension.
> [JDK-8331150](https://bugs.openjdk.org/browse/JDK-8331150) makes this constraint explicit for subword gather load.
> For parity and consistency, this also makes it explicit for subword scatter store as well.
> 
> Testing: `jdk_vector` tested with QEMU vector extension.

Thanks for the review!

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PR Comment: https://git.openjdk.org/jdk/pull/24787#issuecomment-2822882676