[jdk20u] RFR: 8290822: C2: assert in PhaseIdealLoop::do_unroll() is subject to undefined behavior

[Tobias Hartmann]

Backport of [JDK-8290822](https://bugs.openjdk.java.net/browse/JDK-8290822). Applies cleanly. Approval is pending.

Thanks,
Tobias

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Commit messages:
 - 8290822: C2: assert in PhaseIdealLoop::do_unroll() is subject to undefined behavior

Changes: https://git.openjdk.org/jdk20u/pull/41/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk20u&pr=41&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8290822
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk20u/pull/41.diff
  Fetch: git fetch https://git.openjdk.org/jdk20u.git pull/41/head:pull/41

PR: https://git.openjdk.org/jdk20u/pull/41