Now that we have scalable vectors, there's a distinction that isn't getting captured in the original SequentialType: some vectors don't have a known element count, so counting the number of elements doesn't make sense.
In some cases, there's a better way to express the commonality using other methods. If we're dealing with GEPs, there's GEP methods; if we're dealing with a ConstantDataSequential, we can query its element type directly.
In the relatively few remaining cases, I just decided to write out the type checks. We're talking about relatively few places, and I think the abstraction doesn't really carry its weight. (See thread "[RFC] Refactor class hierarchy of VectorType in the IR" on llvmdev.)