RFR: 8373513: C2: Move ProjNode::other_if_proj() to IfProjNode [v2]

[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

Christian Hagedorn has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains three additional commits since the last revision:

 - Fix nullptr
 - Merge branch 'master' into JDK-8373513
 - 8373513: C2: Move ProjNode::other_if_proj() to IfProjNode

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/28785/files
  - new: https://git.openjdk.org/jdk/pull/28785/files/914be8ce..16192544

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=28785&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=28785&range=00-01

  Stats: 14019 lines in 551 files changed: 9174 ins; 1583 del; 3262 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