RFR: 8373524: C2: no reachable node should have no use [v3]

[Christian Hagedorn]

On Wed, 17 Dec 2025 14:16:29 GMT, Roland Westrelin <roland at openjdk.org> wrote:

>> The failure occurs because `PhiNode::Ideal` uses `set_req` to update
>> an input of a `Phi`. That causes the previous input to be disconnected
>> but because of the use of `set_req`, the previous input that has no
>> use is not enqueued for `igvn` to be reclaimed. The fix is to use
>> `set_req_X` instead. I replaced uses of `set_req` with `set_req_X` in
>> `PhiNode::Ideal` where I thought it made sense.
>
> Roland Westrelin has updated the pull request incrementally with one additional commit since the last revision:
> 
>   review

Update looks good, thanks!

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Marked as reviewed by chagedorn (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/28841#pullrequestreview-3597482122