[riscv-port-jdk17u:riscv-port] RFR: 8290137: riscv: small refactoring for add_memory_int32/64

[Dingli Zhang]

Please review this backport to riscv-port-jdk17u.
Backport of [JDK-8290137](https://bugs.openjdk.org/browse/JDK-8290137). Applies cleanly.

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

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Commit messages:
 - 8290137: riscv: small refactoring for add_memory_int32/64

Changes: https://git.openjdk.org/riscv-port-jdk17u/pull/34/files
 Webrev: https://webrevs.openjdk.org/?repo=riscv-port-jdk17u&pr=34&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290137
  Stats: 87 lines in 8 files changed: 40 ins; 0 del; 47 mod
  Patch: https://git.openjdk.org/riscv-port-jdk17u/pull/34.diff
  Fetch: git fetch https://git.openjdk.org/riscv-port-jdk17u.git pull/34/head:pull/34

PR: https://git.openjdk.org/riscv-port-jdk17u/pull/34