8332153: RISC-V: enable tests and add comment for vector shift instruct (shared by vectorization and Vector API)

[Hamlin-Li]

Hi, Can you help to review this patch? For vector shift instruct, some corresponding tests are not enabled, this is to enable them. And the way how vector shift instruct works is not clear, especially both vectorization (SLP in jdk) and Vector API share the same instruct's in riscv_v.ad, so also added some comment to clarify it.

Thanks

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• JDK-8332153: RISC-V: enable tests and add comment for vector shift instruct (shared by vectorization and Vector API) (Enhancement - P4)

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