RFR: 8351392: C2 crash: failed: Expected Bool, but got OpaqueMultiversioning

[Emanuel Peter]

With https://github.com/openjdk/jdk/pull/23865 I mark the `OpaqueMultiversioning` useless once there are no loops for the multiversioning any more. The idea was that this way the `multiversion_if` would be folded away once the loops are gone, and then we should not encounter the `OpaqueMultiversioning` in `PhaseIdealLoop::conditional_move`.

But there is a case where we do not remove the `OpaqueMultiversioning` fast enough, see the attached regression test:
- The loops disappear during IGVN.
- At the beginning of the next loop-opts we mark the `OpaqueMultiversioning` as useless.
- Later during loop-opts we encounter the useless `OpaqueMultiversioning` in `PhaseIdealLoop::conditional_move`, and assert.
- But in the IGVN after this loop-opts phase we would have constant folded the `OpaqueMultiversioning` and `multiversion_if` anyway... we just did not get there fast enough ;)

Hence I propose to just create an explicit bailout for useless `OpaqueMultiversioning` nodes in `PhaseIdealLoop::conditional_move`.

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Commit messages:
 - the fix
 - JDK-8351392

Changes: https://git.openjdk.org/jdk/pull/23943/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23943&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8351392
  Stats: 32 lines in 3 files changed: 32 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/23943.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23943/head:pull/23943

PR: https://git.openjdk.org/jdk/pull/23943