[riscv-port-jdk11u:riscv-port] RFR: 8290137: riscv: small refactoring for add_memory_int32/64

[Dingli Zhang]

Please review this backport to riscv-port-jdk11u.
Backport of [JDK-8290137](https://bugs.openjdk.org/browse/JDK-8290137). Applies not cleanly because [JDK-8339248](https://bugs.openjdk.org/browse/JDK-8339248) remove li64 macro assembler routine and related code. 

So I removed the changes of the li64 macro assembler routine. In addition to this there was an extra modification to `add_memory_int32` in `c1_MacroAssembler_riscv.cpp`.

Testing:
- Tier1 passed without new failure on lp4a (release).

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

Changes: https://git.openjdk.org/riscv-port-jdk11u/pull/30/files
  Webrev: https://webrevs.openjdk.org/?repo=riscv-port-jdk11u&pr=30&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290137
  Stats: 66 lines in 9 files changed: 40 ins; 0 del; 26 mod
  Patch: https://git.openjdk.org/riscv-port-jdk11u/pull/30.diff
  Fetch: git fetch https://git.openjdk.org/riscv-port-jdk11u.git pull/30/head:pull/30

PR: https://git.openjdk.org/riscv-port-jdk11u/pull/30