RFR: 8344251: C2: remove blackholes with dead control input

[Vladimir Kozlov]

On Tue, 15 Apr 2025 13:39:52 GMT, Marc Chevalier <mchevalier at openjdk.org> wrote:

> 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.

C2 fix code is good.

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

Marked as reviewed by kvn (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/24663#pullrequestreview-2769036860