RFR: 8373513: C2: Move ProjNode::other_if_proj() to IfProjNode

[Roland Westrelin]

On Fri, 12 Dec 2025 09:48:28 GMT, Christian Hagedorn <chagedorn at openjdk.org> wrote:

> This is a simple clean-up patch which moves `ProjNode::other_if_proj()` to `IfProjNode` and update its uses. It only makes sense to call `other_if_proj()` on actual `IfProjNodes`.
> 
> It also required to update more types from `ProjNode` to `IfProjNode` which is more type-safe and preciser. While touching the methods, I've also added some `const`/`static` where appropriate.
> 
> Thanks,
> Christian

Looks good to me.

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Marked as reviewed by roland (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/28785#pullrequestreview-3571917991