[lworld] RFR: 8315744: [lworld] SubTypeCheckNode::sub asserts with "should be not null"

[Tobias Hartmann]

[JDK-8303279](https://bugs.openjdk.org/browse/JDK-8303279) added an assert that we now hit in Valhalla because the subtype of a `SubTypeCheckNode` is not known to be non-null. The problem is that when pushing InlineTypeNodes through Phis, we lose track of the fact that the Phi is non-null. There's some existing logic for that but it's broken.

Thanks,
Tobias

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Commit messages:
 - 8315744: [lworld] SubTypeCheckNode::sub asserts with "should be not null"

Changes: https://git.openjdk.org/valhalla/pull/944/files
 Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=944&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315744
  Stats: 15 lines in 2 files changed: 0 ins; 9 del; 6 mod
  Patch: https://git.openjdk.org/valhalla/pull/944.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/944/head:pull/944

PR: https://git.openjdk.org/valhalla/pull/944