[jdk19u] RFR: 8293996: C2: fix and simplify IdealLoopTree::do_remove_empty_loop

[Tobias Hartmann]

Backport of [JDK-8293996](https://bugs.openjdk.java.net/browse/JDK-8293996). Applies cleanly. Approval is pending.

Thanks,
Tobias

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Commit messages:
 - 8293996: C2: fix and simplify IdealLoopTree::do_remove_empty_loop

Changes: https://git.openjdk.org/jdk19u/pull/86/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk19u&pr=86&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293996
  Stats: 65 lines in 2 files changed: 38 ins; 17 del; 10 mod
  Patch: https://git.openjdk.org/jdk19u/pull/86.diff
  Fetch: git fetch https://git.openjdk.org/jdk19u pull/86/head:pull/86

PR: https://git.openjdk.org/jdk19u/pull/86