RFR: 8296771: RISC-V: C2: assert(false) failed: bad AD file

[Xiaolin Zheng]

The backend encounters the same assertion failure as [JDK-8295414](https://bugs.openjdk.org/browse/JDK-8295414). This patch is a similar fix.

Same, `partialSubtypeCheck` uses `iRegP_R15` as a result while its use `iRegP`  doesn't match the `iRegP_R15`, causing this failure.

The details are in the JBS issue [JDK-8296771](https://bugs.openjdk.org/browse/JDK-8296771).

Tested the failed `compiler/types/TestSubTypeCheckMacroTrichotomy.java`, and a hotspot tier1 is running now.

Thanks,
Xiaolin

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Commit messages:
 - Fix simply

Changes: https://git.openjdk.org/jdk/pull/11085/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11085&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296771
  Stats: 1 line in 1 file changed: 1 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/11085.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11085/head:pull/11085

PR: https://git.openjdk.org/jdk/pull/11085