RFR: 8344251: C2: remove blackholes with dead control input [v3]

[Marc Chevalier]

> When a BlackholeNode's control input becomes dead, the node is not removed causing the crash
> 
>  assert(!in->is_CFG()) failed: CFG Node with no controlling input?
> 
> In the case reported in the issue, after a round of peeling, a condition becomes constant, and the branch containing the blackhole becomes dead:
> 
> <img src="https://github.com/user-attachments/assets/f8e5ec85-ddf5-4018-a01e-30ea90d9d74f" width="400">
> <img src="https://github.com/user-attachments/assets/626e40a1-be2f-4b1c-bc56-9760a26660f2" width="400">
> 
> I simply use `Node::remove_dead_region(PhaseGVN*, bool)` to remove the blackhole, as many other node types do.

Marc Chevalier has updated the pull request incrementally with one additional commit since the last revision:

  cosmetic

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/24663/files
  - new: https://git.openjdk.org/jdk/pull/24663/files/487b1a64..9f000990

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=24663&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=24663&range=01-02

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/24663.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24663/head:pull/24663

PR: https://git.openjdk.org/jdk/pull/24663