RFR: 8347555: [REDO] C2: implement optimization for series of Add of unique value [v7]

[Kangcheng Xu]

On Wed, 2 Apr 2025 10:08:11 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> Ah interesting. It could be worth adding a comment for that here then!
>
> That was in fact a large part of my initial hesitation with this PR.

Added comments on `(a*b) + (a*c)` and `AddNode::IdealIL`

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PR Review Comment: https://git.openjdk.org/jdk/pull/23506#discussion_r2038113619