[LoopVectorize] Implement check-first vectorization with scalar replay

Implement the core check-first vectorization strategy for simple loops and loops with cascaded early exits.
The transformation evaluates exit conditions before executing the vectorized loop body. When an exit is detected, execution continues through scalar replay.

High level objectives:
1. Add legality analysis for check-first vectorization.
2. Construct the check-first VPlan.
3. Support single and cascaded exits.
4. Wire early exits to scalar replay.
5. Add the required cost-model changes.
6. Add tests for single exits, cascaded exits, instruction reordering, and state updates.

Depends on [#215468](https://github.com/llvm/llvm-project/issues/215468)

Start by reviewing prerequisite issue #215468, then trace LLVM's legality analysis, VPlan construction, early-exit and scalar-replay handling, and cost-model work. Done means supporting simple loops and cascaded early exits, with tests covering exits, instruction reordering, and state updates.

Written by the indexing model from the issue text.