RFR: 8367066: RISC-V: refine register selection in MacroAssembler:: decode_klass_not_null

[Hamlin Li]

Hi,
Can you help to review this patch?

Previously, the register selection in MacroAssembler:: decode_klass_not_null is misleading, better to refine it to improve the readability.

Thanks!

Running runtime test tier1/2/3...

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Commit messages:
 - initial commit

Changes: https://git.openjdk.org/jdk/pull/27138/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27138&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8367066
  Stats: 2 lines in 1 file changed: 1 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/27138.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27138/head:pull/27138

PR: https://git.openjdk.org/jdk/pull/27138