This patch adds support for a user-defined pass-pipeline that overrides
the default pipeline of the vectorizer.
This will commonly be used by lit tests.
This patch implements a new empty pass for the Bottom-up vectorizer and
creates a pass pipeline that includes it.
The SandboxVectorizer LLVM pass runs the Sandbox IR pass pipeline.
This patch implements the new pass and registers it with the pass
manager. For context, this is a vectorizer that operates on Sandbox IR,
which is a transactional IR on top of LLVM IR.
