RFR: 8353290: C2: Refactor PhaseIdealLoop::is_counted_loop() [v17]

[Kangcheng Xu]

> This PR refactors `PhaseIdealLoop::is_counted_loop()` into (mostly) `CountedLoopConverter::is_counted_loop()` and `CountedLoopConverter::convert()` to decouple the detection and conversion code. This enables us to try different loop configurations easily and finally convert once a counted loop is found. 
> 
> A nested `PhaseIdealLoop::CountedLoopConverter` class is created to handle the context, but I'm not if this is the best name or place for it. Please let me know what you think.
> 
> Blocks [JDK-8336759](https://bugs.openjdk.org/browse/JDK-8336759).

Kangcheng Xu has updated the pull request incrementally with one additional commit since the last revision:

  mark LoopExitTest::is_valid_with_bt() const

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Changes:
  - all: https://git.openjdk.org/jdk/pull/24458/files
  - new: https://git.openjdk.org/jdk/pull/24458/files/9005b864..0a3fff1b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=24458&range=16
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=24458&range=15-16

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/24458.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24458/head:pull/24458

PR: https://git.openjdk.org/jdk/pull/24458