RISC-V: Refactor UseRVV related checks

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 get the final UseRVV value instead of inital value.