RFR: 8370666: VectorAPI: Add clear comments for vector relative code in c2 [v4]

[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.

Xiaohong Gong has updated the pull request incrementally with one additional commit since the last revision:

  Update comments in vectornode.hpp

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/29130/files
  - new: https://git.openjdk.org/jdk/pull/29130/files/083f5754..431f2b86

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=29130&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=29130&range=02-03

  Stats: 12 lines in 1 file changed: 3 ins; 0 del; 9 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