[mlir][linalg] Add getCollapsedVecType and update vectorization of linalg.unpack (#151503)

This patch introduces a new helper, `getCollapsedVecType`, and updates
`vectorizeAsTensorUnpackOp` to use it. The motivation stems from improving how
`vector.shape_cast` operations are generated when vectorizing `linalg.unpack`.

Previously, the vectorizer relied on
* `tensor::CollapseShapeOp::inferCollapsedType`

to compute the collapsed vector type. This approach is suboptimal
because:
  * `inferCollapsedType` lacks awareness of scalable vector flags.
  * Linalg vectorization should not depend on Tensor dialect utilities.

Instead of relocating `inferCollapsedType`, we introduce
`getCollapsedVecType` — a lightweight, specialized hook that:
  * Assumes no dynamic sizes.
  * Handles scalable flags alongside shape dimensions.

This change also reduces temporary variables in
`vectorizeAsTensorUnpackOp` and paves the way for a cleaner update in
 #149293.