RFR: 8355654: RISC-V: Relax register constraint for some vector-scalar instructions

[Gui Cao]

Hi, here we separate src from dst_src to give the compiler a greater degree of flexibility. Note that in the masked version we can't modify it, because we can't change inactive elements in the mask version.

### Testing
qemu-system 9.1.50 with UseRVV:

- [x] Run test/jdk/jdk/incubator/vector (fastdebug)

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Commit messages:
 - 8355654: RISC-V: Relax register constraint for some vector-scalar instructions

Changes: https://git.openjdk.org/jdk/pull/24905/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24905&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8355654
  Stats: 66 lines in 1 file changed: 0 ins; 0 del; 66 mod
  Patch: https://git.openjdk.org/jdk/pull/24905.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24905/head:pull/24905

PR: https://git.openjdk.org/jdk/pull/24905