RFR: 8332153: RISC-V: enable tests and add comment for vector shift instruct (shared by vectorization and Vector API) [v4]

[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

Hamlin Li has updated the pull request incrementally with one additional commit since the last revision:

  minor

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/19265/files
  - new: https://git.openjdk.org/jdk/pull/19265/files/7216d886..2ba613fa

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=19265&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=19265&range=02-03

  Stats: 3 lines in 2 files changed: 0 ins; 1 del; 2 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