RFR: 8371536: C2: VerifyIterativeGVN should assert on first detected failure [v9]

[Benoît Maillard]

On Wed, 28 Jan 2026 10:08:25 GMT, Manuel Hässig <mhaessig at openjdk.org> wrote:

>> Defining a small, say `GVNVerificationPhase`, enum would already be cleaner, safer and less overhead.
>
> You could even make that a debug only field of `PhaseIterGVN` and override it for `PhaseCCP`.

Following a discussion with @mhaessig, I decided to replace the `_iterGVN` boolean field by an enum field, and refactor `is_IterGVN` using this new field. 

```c++
enum class PhaseValuesType {
    gvn,
    iter_gvn,
    ccp
  };

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PR Review Comment: https://git.openjdk.org/jdk/pull/28295#discussion_r2737650119