RFR: 8353290: C2: Refactor PhaseIdealLoop::is_counted_loop()

[Kangcheng Xu]

On Fri, 4 Apr 2025 20:33:47 GMT, Kangcheng Xu <kxu at openjdk.org> wrote:

> This PR refactors `PhaseIdealLoop::is_counted_loop()` into (mostly) `CountedLoopConverter::is_counted_loop()` and `CountedLoopConverter::convert()` to decouple to detection and conversion code. This enables us to try different loop configurations easy 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).

C2 changes

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PR Comment: https://git.openjdk.org/jdk/pull/24458#issuecomment-2783724710