[mlir][vector] Relax constraints on shape_cast (#136587)

`vector.shape_cast` was initially designed to be the union of
collapse_shape and expand_shape. There was an inconsistency in the
verifier that allowed any shape casts when the rank did not change, which
led to a strange middle ground where you could cast from shape (4,3) to
(3,4) but not from (4,3) to (2,3,2). That issue was fixed (verifier made stricter) 
in https://github.com/llvm/llvm-project/pull/135855, but further feedback
there (and polling) suggests that vector.shape_cast should rather allow all 
shape casts (so more like tensor.reshape than 
tensor.collapse_shape/tensor.expand_shape). This PR makes this simplification
by relaxing the verifier.

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