[riscv-port-jdk17u:riscv-port] RFR: 8294430: RISC-V: Small refactoring for movptr_with_offset

[Dingli Zhang]

Please review this backport to riscv-port-jdk17u.
Backport of [JDK-8294430](https://bugs.openjdk.org/browse/JDK-8294430).
The original patch cannot be directly applied because jdk17u has no `emit_etnry_barrier_stub`, which introduce with [JDK-8293007](https://bugs.openjdk.org/browse/JDK-8293007) , and there are no other additional changes.

Testing:
- Tier1-3 passed without new failure on unmacthed (release).

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Commit messages:
 - 8294430: RISC-V: Small refactoring for movptr_with_offset

Changes: https://git.openjdk.org/riscv-port-jdk17u/pull/36/files
 Webrev: https://webrevs.openjdk.org/?repo=riscv-port-jdk17u&pr=36&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8294430
  Stats: 25 lines in 9 files changed: 0 ins; 2 del; 23 mod
  Patch: https://git.openjdk.org/riscv-port-jdk17u/pull/36.diff
  Fetch: git fetch https://git.openjdk.org/riscv-port-jdk17u.git pull/36/head:pull/36

PR: https://git.openjdk.org/riscv-port-jdk17u/pull/36