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

[Xiaohong Gong]

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.

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Commit messages:
 - 8370666: VectorAPI: Add clear comments for vector mask relative code in c2

Changes: https://git.openjdk.org/jdk/pull/29130/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29130&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8370666
  Stats: 149 lines in 5 files changed: 129 ins; 2 del; 18 mod
  Patch: https://git.openjdk.org/jdk/pull/29130.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29130/head:pull/29130

PR: https://git.openjdk.org/jdk/pull/29130