RFR: 8370220: C2: rename methods and improve documentation around get_ctrl and idom lazy updating/forwarding of ctrl and idom via dead ctrl nodes

[Emanuel Peter]

When working on https://github.com/openjdk/jdk/pull/27889, I was irritated by the lack of documentation and suboptimal naming.

Here, I'm doing the following:
- Add more documentation, and improve it in other cases.
- Rename "lazy" methods: "lazy" could indicate that we delay it somehow until later, but it is unclear what is delayed.
  - `lazy_replace` -> `replace_ctrl_node_and_forward_ctrl_and_idom`
  - `lazy_update` -> `install_lazy_ctrl_and_idom_forwarding`
- Made some methods private, and added some additional asserts.

I'd be more than happy for even better names, and suggestions how to improve the documentation further :)

Related issues:
https://github.com/openjdk/jdk/pull/27889
https://github.com/openjdk/jdk/pull/15720

TODO: improve `VerifyLoopOptimizations` to check that we can call `get_ctrl` on all live nodes after loop-opts.

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Commit messages:
 - code style
 - missing part
 - rename lazy methods
 - make helper method private
 - wip documentation and renaming
 - more documentation
 - JDK-8370220

Changes: https://git.openjdk.org/jdk/pull/27892/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27892&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8370220
  Stats: 103 lines in 6 files changed: 53 ins; 4 del; 46 mod
  Patch: https://git.openjdk.org/jdk/pull/27892.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27892/head:pull/27892

PR: https://git.openjdk.org/jdk/pull/27892