[Refactor][BugFix] Refactor the loop vectorization plan with ConstraintKind (#2935)

* [BugFix] Preserve invariant store scalarization

Classify vectorization constraints explicitly so broadcast loads, memory accesses, local accesses, and semantic scalarization requirements cannot be conflated by scope-based bucketing.

* [Refactor] Preserve vectorization planner diagnostics

Move the original strategy rationale into the new classification helpers and retain the established verbose summary and strategy log messages while adding separate constraint details.

* [Docs] Restore vectorization strategy overview