[jdk23u] RFR: 8340590: RISC-V: C2: Small improvement to vector gather load and scatter store

[Gui Cao]

Hi, The same issue also exists in the jdk23u. I would like to backport 8340590 to jdk23u. This is a risc-v specific change. Backport is clean, risk is low.

### Testing
- [ ] make test TEST="jdk_vector" JTREG="TIMEOUT_FACTOR=32" on qemu with UseRVV1.0

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Commit messages:
 - Backport 88801caef6ccdc5ba9ade2af830f3b3cd96e1467

Changes: https://git.openjdk.org/jdk23u/pull/141/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk23u&pr=141&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340590
  Stats: 8 lines in 1 file changed: 0 ins; 4 del; 4 mod
  Patch: https://git.openjdk.org/jdk23u/pull/141.diff
  Fetch: git fetch https://git.openjdk.org/jdk23u.git pull/141/head:pull/141

PR: https://git.openjdk.org/jdk23u/pull/141