RFR: 8340590: RISC-V: C2: Small improvement to vector gather load and scattter store [v2]

[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 qemu with UseRVV1.0

Gui Cao has updated the pull request incrementally with one additional commit since the last revision:

  Polishing

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Changes:
  - all: https://git.openjdk.org/jdk/pull/21123/files
  - new: https://git.openjdk.org/jdk/pull/21123/files/7f1cb3ec..03f6e3a8

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21123&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21123&range=00-01

  Stats: 8 lines in 1 file changed: 0 ins; 4 del; 4 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