[lworld] RFR: 8257422: [lworld] Problems with scalarized inline type return and incremental inlining

[Tobias Hartmann]

TestCallingConvention spuriously fails with -Xcomp and -XX:-TieredCompilation due to incremental inlining not properly handling scalarized inline type returns in rare cases. The actual fix is in parse1.cpp where we accidentally use an inline type PhiNode to return an inline type ptr when incrementally inlining (in some cases, that also manifests as crashes during compilation).

I've also refactored some related code, added stronger asserts, improved comments and added tests that reproduce the issue more reliably.

Thanks,
Tobias

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Commit messages:
 - 8257422: [lworld] Problems with scalarized inline type return and incremental inlining

Changes: https://git.openjdk.java.net/valhalla/pull/285/files
 Webrev: https://webrevs.openjdk.java.net/?repo=valhalla&pr=285&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257422
  Stats: 98 lines in 5 files changed: 67 ins; 5 del; 26 mod
  Patch: https://git.openjdk.java.net/valhalla/pull/285.diff
  Fetch: git fetch https://git.openjdk.java.net/valhalla pull/285/head:pull/285

PR: https://git.openjdk.java.net/valhalla/pull/285