RFR: 8370666: VectorAPI: Add clear comments for vector mask relative code in c2

[Quan Anh Mai]

On Fri, 9 Jan 2026 01:36:50 GMT, Xiaohong Gong <xgong at openjdk.org> wrote:

> The VectorMask implementation in Vector API involves complex interactions between types, nodes, and platform-specific
> features, making the related code in HotSpot difficult to understand and review.
> 
> This patch adds comprehensive comments for vector mask related types, nodes, and methods in C2 to improve code clarity and
> maintainability.
> 
> Note: This patch only adds comments; no functional changes are made.

Marked as reviewed by qamai (Committer).

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PR Review: https://git.openjdk.org/jdk/pull/29130#pullrequestreview-3649886055