RFR: 8354930: IGV: dump C2 graph before and after live range stretching

[Christian Hagedorn]

On Wed, 28 May 2025 11:54:24 GMT, Manuel Hässig <mhaessig at openjdk.org> wrote:

> This PR introduces a new phase `LIVE_RANGE_STRETCHING` that prints after live ranges have been stretched, if that happens at all. The phase `INITIAL_LIVENESS` is moved before live range stretching so we can compare the live ranges before and after stretching in IGV, which is useful for debugging why an oop suddenly belongs to an oop map.
> 
> ## Testing
> 
>  - [x] [Github Actions](https://github.com/mhaessig/jdk/actions/runs/15299362485)
>  - [x] tier1 and tier1, plus additional Oracle internal testing for all Oracle supported platforms and OSs
>  - [x] verified that the new phase prints when it should in IGV and with `-XX:PrintPhaseLevel=4`

Looks good to me, too.

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Marked as reviewed by chagedorn (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/25492#pullrequestreview-2882457988