[jdk17u-dev] RFR: 8293996: C2: fix and simplify IdealLoopTree::do_remove_empty_loop

[Goetz Lindenmaier]

I backport this for parity with 17.0.7-oracle.

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Commit messages:
 - Backport dd51f7e0b75d3a16403608d89cd206ac0bedf882

Changes: https://git.openjdk.org/jdk17u-dev/pull/998/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk17u-dev&pr=998&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/jdk17u-dev/pull/998.diff
  Fetch: git fetch https://git.openjdk.org/jdk17u-dev pull/998/head:pull/998

PR: https://git.openjdk.org/jdk17u-dev/pull/998