RFR: 8370220: C2: rename methods and improve documentation around get_ctrl and idom lazy updating/forwarding of ctrl and idom via dead ctrl nodes [v3]

[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.

Emanuel Peter has updated the pull request incrementally with one additional commit since the last revision:

  fix merge

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/27892/files
  - new: https://git.openjdk.org/jdk/pull/27892/files/c3ab0698..e0b6ad68

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=27892&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=27892&range=01-02

  Stats: 8 lines in 2 files changed: 1 ins; 0 del; 7 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