RFR: 8328107: Shenandoah/C2: TestVerifyLoopOptimizations test failure

[Roland Westrelin]

The failure occurs because a load barrier is expanded on the backedge
of the counted loop. That breaks the expected counted loop shape. The
fix I propose is to replace the `CountedLoop` with a `Loop` node when
that happens. We're basically done with optimizations related to
counted loop at this point so this shouldn't make a difference.

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Commit messages:
 - test and fix

Changes: https://git.openjdk.org/jdk/pull/19259/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19259&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8328107
  Stats: 92 lines in 2 files changed: 92 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/19259.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19259/head:pull/19259

PR: https://git.openjdk.org/jdk/pull/19259