[SandboxVec] Add a simple pack reuse pass (#141848)
This patch implements a simple pass that tries to de-duplicate packs. If
there are two packing patterns inserting the exact same values in the
exact same order, then we will keep the top-most one of them. Even
though such patterns may be optimized away by subsequent passes it is
still useful to do this within the vectorizer because otherwise the cost
estimation may be off, making the vectorizer over conservative.
