RFR: 8370666: VectorAPI: Add clear comments for vector relative code in c2 [v3]

[Xiaohong Gong]

On Fri, 16 Jan 2026 08:51:01 GMT, Emanuel Peter <epeter at openjdk.org> wrote:

>> I filed it here with an example:
>> [JDK-8375502](https://bugs.openjdk.org/browse/JDK-8375502) C2 SuperWord: implement unsigned casts
>> 
>> If anybody is interested in taking on this task, feel free to reassign it to yourself :)
>
> I'd suggest to remove the comment about "Vector API only" in the code. Instead, I would explain that unsigned cast means zero-extension if dst is larger than src.

Sounds good. I will change it with next commit. Thanks for the suggestion!

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PR Review Comment: https://git.openjdk.org/jdk/pull/29130#discussion_r2697690358