[SandboxVec][NFC] Lit tests cleanup (#214794)
This patch organizes the lit tests by introducing a directory structure,
separating them by pass, and renaming them when needed. This will become
increasingly useful for navigating through the lit tests as we keep
adding more tests and more passes.
This also drops boilerplate.ll as it was one of the early tests that
checked the vectorizer's boilerplate and is no longer needed.
