RFR: 8339992: RISC-V: some minor improvements of base64_vector_decode_round

[Hamlin Li]

Hi,
Can you help to review this simple improvement?
Thanks.

-------------

Commit messages:
 - comments
 - initial commit

Changes: https://git.openjdk.org/jdk/pull/21059/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21059&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339992
  Stats: 9 lines in 1 file changed: 4 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/21059.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21059/head:pull/21059

PR: https://git.openjdk.org/jdk/pull/21059