RFR: 8367690: C2: Unneeded branch in reduce_phi

[Anton Seoane Ampudia]

On Thu, 16 Oct 2025 14:49:02 GMT, Anton Seoane Ampudia <aseoane at openjdk.org> wrote:

> This PR carries out a minor cleanup found in the Phi reduction code.
> 
> The combination of the branch and assert is redundant as the assert will always trigger. We can either remove the `else if` branch or change the assert for a `ShouldNotReachHere`.
> 
> I have gone for the former option here.
> 
> **Testing:** passes tiers 1-3

Thanks all!

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PR Comment: https://git.openjdk.org/jdk/pull/27849#issuecomment-3431208509