[jdk21u-dev] RFR: 8340590: RISC-V: C2: Small improvement to vector gather load and scatter store

[Gui Cao]

Hi, The same issue also exists in the jdk21u. I would like to backport 8340590 to jdk21u-dev. 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

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

Commit messages:
 - Backport 88801caef6ccdc5ba9ade2af830f3b3cd96e1467

Changes: https://git.openjdk.org/jdk21u-dev/pull/1033/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=1033&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/jdk21u-dev/pull/1033.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/1033/head:pull/1033

PR: https://git.openjdk.org/jdk21u-dev/pull/1033