RFR: 8346774: Use Predicate classes instead of Node classes

[Christian Hagedorn]

This small cleanup PR replaces a lot of usages of `Node` pointers, to pass around either the head (i.e. `IfNode`) or the tail (i.e. a success projection) of predicates, with actual `Predicate` classes. This simplifies the usages, readability and the logical flow, and enables more simplifications in the future, especially once we replace Template Assertion Predicates with a dedicated node.

I've also included some minor refactorings like adding `const` or fixing typos.

There are no semantic changes involved. The return value optimization should take care to avoid a lot of copies when returning new objects from methods. 

Thanks,
Christian

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Commit messages:
 - more cleanups
 - more cleanups
 - 8346774: Use Predicate classes instead of Node classes

Changes: https://git.openjdk.org/jdk/pull/23234/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23234&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8346774
  Stats: 123 lines in 2 files changed: 28 ins; 4 del; 91 mod
  Patch: https://git.openjdk.org/jdk/pull/23234.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23234/head:pull/23234

PR: https://git.openjdk.org/jdk/pull/23234