RFR: 8355654: RISC-V: Relax register constraint for some vector-scalar instructions [v4]

[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)

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/24905/files
  - new: https://git.openjdk.org/jdk/pull/24905/files/c00c40fb..4fa2fc89

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=24905&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=24905&range=02-03

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