[riscv-port-jdk11u:riscv-port] RFR: 8284937: riscv: should not allocate special register for temp

[Gui Cao]

Hi, please help review this backport to riscv-port-jdk11u.
Backport of [JDK-8284937](https://bugs.openjdk.org/browse/JDK-8284937). The original patch cannot be directly applied because riscv-port-jdk11u doesn't have riscv_v.ad and doesn't have c2_MacroAssembler, which uses MacroAssembler.

Testing:

- [x] Run tier1-3 tests on SOPHON SG2042 (release)

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Commit messages:
 - backport 8284937: riscv: should not allocate special register for temp

Changes: https://git.openjdk.org/riscv-port-jdk11u/pull/17/files
  Webrev: https://webrevs.openjdk.org/?repo=riscv-port-jdk11u&pr=17&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8284937
  Stats: 28 lines in 1 file changed: 0 ins; 0 del; 28 mod
  Patch: https://git.openjdk.org/riscv-port-jdk11u/pull/17.diff
  Fetch: git fetch https://git.openjdk.org/riscv-port-jdk11u.git pull/17/head:pull/17

PR: https://git.openjdk.org/riscv-port-jdk11u/pull/17