[mlir][vector] Add vector.step operation (#96776)

This patch adds a new vector.step operation to the Vector dialect. It
produces a linear sequence of index values from 0 to N, where N is the
number of elements in the result vector, and can be used to create
vectors of indices.

It supports both fixed-width and scalable vectors. For fixed the
canonical representation is `arith.constant dense<[0, .., N]>`. A
scalable step cannot be represented as a constant and is lowered to the
`llvm.experimental.stepvector` intrinsic [1].

This op enables scalable vectorization of linalg.index ops, see #96778. It can
also be used in the SparseVectorizer in-place of lower-level stepvector
intrinsic, see [2] (patch to follow).

[1] https://llvm.org/docs/LangRef.html#llvm-experimental-stepvector-intrinsic
[2] https://github.com/llvm/llvm-project/blob/acf675b63f9426e61aac2155e29280f7d21f9421/mlir/lib/Dialect/SparseTensor/Transforms/SparseVectorization.cpp#L385-L388

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