RFR: 8350756: C2 SuperWord Multiversioning: remove useless slow loop when the fast loop disappears

[Emanuel Peter]

@rwestrel asked me for this here:
https://github.com/openjdk/jdk/pull/22016#issuecomment-2684365921

The idea: an unused `multiversion_if` (i.e. one where the `slow_loop` is still delayed, i.e. where the `fast_loop` has not yet added a runtime-check to the `multiversion_if`) can be constant-folded if the main `fast_loop` disappears. Because at that point we know that we will never add a new condition to the `multiversion_if`, and it will constant fold to true (towards the `fast_loop`) after loop-opts anyway.

It also seems to fix a bug, where all multiversioned loops (fast / slow, pre/main/post) disappear, and then we are left with a if-diamond with the multiversion_if. This then hits assertion code in `PhaseIdealLoop::conditional_move`. This issue is also addressed with this patch here. I adjusted the code around that assert slightly to give better reporting and ensure we bail out of the optimization if we see an unexpected pattern.

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Commit messages:
 - fix assert for VerifyLoopOpts
 - whitespace
 - added test
 - improve assert
 - JDK-8350756

Changes: https://git.openjdk.org/jdk/pull/23865/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23865&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8350756
  Stats: 156 lines in 6 files changed: 153 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/23865.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23865/head:pull/23865

PR: https://git.openjdk.org/jdk/pull/23865