RFR: 8319781: RISC-V: Refactor UseRVV related checks

[Hamlin Li]

Hi,
Can you review the patch to refactor the code related UseRVV checks?
Thanks!

There are some code (flag setting/checking) depending on UseRVV's value, these code should be refactored, especially after the change of https://bugs.openjdk.org/browse/JDK-8319408:
1. some code needs to get the final UseRVV's value rather than the initial value, e.g. ChaCha20 intrinsic.
2. refactored to be more readable.
3. also add note to make sure the future code does not make the similar mistakes.

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.org/jdk/pull/16580/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16580&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8319781
  Stats: 45 lines in 2 files changed: 18 ins; 24 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/16580.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16580/head:pull/16580

PR: https://git.openjdk.org/jdk/pull/16580