RFR: 8340643: RISC-V: Small refactoring for sub/subw macro-assembler routines

[Gui Cao]

Hi, please help review that, small refactoring for sub/subw macro-assembler routines.

### Testing
- [x] Run tier1 tests on SOPHON SG2042 (release)

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Commit messages:
 - Polishing
 - Polishing
 - 8340643: RISC-V: Small refactoring for sub/subw macro-assembler routines

Changes: https://git.openjdk.org/jdk/pull/21135/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21135&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340643
  Stats: 14 lines in 1 file changed: 0 ins; 12 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/21135.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21135/head:pull/21135

PR: https://git.openjdk.org/jdk/pull/21135