RFR: 8361836: RISC-V: Relax min vector length to 32-bit for short vectors

[Dingli Zhang]

Follow up [JDK-8359419](https://bugs.openjdk.org/browse/JDK-8359419), RVV supports all vector type conversion APIs in the Vector API.
So we only need to relax the length limit of the short type to achieve a significant improvement in JMH performance for converting between short and wider types.

### Test
qemu-system UseRVV:
* [x]  Run jdk_vector (fastdebug)
* [x]  Run compiler/vectorapi (fastdebug)

### Performance
Following shows the performance improvement of relative VectorAPI JMHs on k1 (256-bit RVV):


Benchmark                                             (SIZE)   Mode   Units   Before     After    Gain
VectorFPtoIntCastOperations.microDouble128ToShort128     512  thrpt  ops/ms   52.280   840.112   16.07
VectorFPtoIntCastOperations.microDouble128ToShort128    1024  thrpt  ops/ms   28.156   429.322   15.25
VectorFPtoIntCastOperations.microFloat64ToShort64        512  thrpt  ops/ms   14.242   479.509   33.67
VectorFPtoIntCastOperations.microFloat64ToShort64       1024  thrpt  ops/ms    6.906   242.690   35.14

PS: `VectorFPtoIntCastOperations.microFloat64ToShort64` is added by [JDK-8359419](https://bugs.openjdk.org/browse/JDK-8359419).

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Commit messages:
 - 8361836: RISC-V: Relax min vector length to 32-bit for short vectors

Changes: https://git.openjdk.org/jdk/pull/26239/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=26239&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8361836
  Stats: 18 lines in 2 files changed: 18 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/26239.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/26239/head:pull/26239

PR: https://git.openjdk.org/jdk/pull/26239