[LoopVectorize] Add VPlan infrastructure for check-first vectorization

Add the foundational VPlan infrastructure required to implement check-first vectorization for loops with multiple early exits.
This includes:
1. Adding the VPlan state and APIs needed by check-first vectorization.
2. Adding a supporting recipe and verifier changes.

Start by locating the existing VPlan state and APIs used for vectorization, then read the supporting recipe and verifier code. Done means adding the check-first vectorization state and APIs, a supporting recipe, and the required verifier changes for loops with multiple early exits.

Written by the indexing model from the issue text.