RFR: 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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Commit messages:
 - add comment
 - enable tests

Changes: https://git.openjdk.org/jdk/pull/19265/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19265&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8332153
  Stats: 178 lines in 11 files changed: 174 ins; 0 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/19265.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19265/head:pull/19265

PR: https://git.openjdk.org/jdk/pull/19265