RFR: 8344387: RISC-V: C2: Improve encoding of LoadNKlass for compact headers

[Hamlin Li]

Hi,
Can you help to review this patch? This is a follow-up of 8340453 on riscv.
Thanks!

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Commit messages:
 - initial commit

Changes: https://git.openjdk.org/jdk/pull/22203/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22203&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8344387
  Stats: 18 lines in 3 files changed: 4 ins; 12 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/22203.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22203/head:pull/22203

PR: https://git.openjdk.org/jdk/pull/22203