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

[Christian Hagedorn]

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

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Commit messages:
 - 8373513: C2: Move ProjNode::other_if_proj() to IfProjNode

Changes: https://git.openjdk.org/jdk/pull/28785/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28785&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373513
  Stats: 56 lines in 8 files changed: 6 ins; 8 del; 42 mod
  Patch: https://git.openjdk.org/jdk/pull/28785.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28785/head:pull/28785

PR: https://git.openjdk.org/jdk/pull/28785