[lworld] RFR: 8288098: [lworld] C2 fails to scalarize value class arguments

[Tobias Hartmann]

C2 fails to scalarize a value class argument because its class is not listed for preloading (because the argument class is equal to the method holder class). We should not rely on the preload attribute here but simply check if the argument class is a loaded value class. Proper mismatch handling will be added by [JDK-8284443](https://bugs.openjdk.org/browse/JDK-8284443).

I also added a regression test and fixed unrelated `TestNullableInlineTypes::test97` which failed due to recent changes.

Thanks,
Tobias

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Commit messages:
 - 8288098: [lworld] C2 fails to scalarize value class arguments

Changes: https://git.openjdk.java.net/valhalla/pull/709/files
 Webrev: https://webrevs.openjdk.java.net/?repo=valhalla&pr=709&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8288098
  Stats: 29 lines in 6 files changed: 11 ins; 12 del; 6 mod
  Patch: https://git.openjdk.java.net/valhalla/pull/709.diff
  Fetch: git fetch https://git.openjdk.java.net/valhalla pull/709/head:pull/709

PR: https://git.openjdk.java.net/valhalla/pull/709