[LoopVectorize] Support nested predicates and guarded stores for check-first vectorization

Extend check-first vectorization to support nested exit predicates and guarded memory operations.

High level objectives:

1. Recognize supported diamond-shaped control flow.
2. Convert diamond guards into exit conditions.
3. Build and attach guards to predicated stores.
4. Handle loads used by guarded exit conditions.
5. Add scalar-replay tests for guarded and nested exits.

Depends on [#215471](https://github.com/llvm/llvm-project/issues/215471)

Start with check-first vectorization and the dependency in issue #215471. Trace how diamond-shaped control flow, nested exit predicates, guarded stores, and loads in exit conditions are handled, then add the scalar-replay tests mentioned for guarded and nested exits. Done means all five listed objectives are supported and covered by those tests.

Written by the indexing model from the issue text.