RFR: 8340590: RISC-V: C2: Small improvement to vector gather load and scattter store

[Gui Cao]

Hi, 
This is a small improvement for RISC-V C2 vector gather load and scattter store nodes. Currently, we emit whole vector register move (vmv1r.v) to move vector idx to a temp vector register. But a normal vector integer move (vmv.v.v) would do and is more reasonable as the vtype and vl are known here.


### Testing
- [x] make test TEST="jdk_vector" JTREG="TIMEOUT_FACTOR=32" on Banana Pi BPI-F3 board (with RVV1.0)

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Commit messages:
 - 8340590: RISC-V: C2: Small improvement to vector gather load and scattter store

Changes: https://git.openjdk.org/jdk/pull/21123/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21123&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340590
  Stats: 8 lines in 1 file changed: 4 ins; 4 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/21123.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21123/head:pull/21123

PR: https://git.openjdk.org/jdk/pull/21123