[jdk21u-dev] RFR: 8331575: C2: crash when ConvL2I is split thru phi at LongCountedLoop

[Martin Doerr]

Clean backport of [JDK-8331575](https://bugs.openjdk.org/browse/JDK-8331575).

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Commit messages:
 - Backport f398cd225012694a586e528936159b6df7b1586c

Changes: https://git.openjdk.org/jdk21u-dev/pull/719/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=719&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8331575
  Stats: 129 lines in 3 files changed: 127 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/719.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/719/head:pull/719

PR: https://git.openjdk.org/jdk21u-dev/pull/719