[MLIR][XeGPU] Add handling for unit-dim expansion in ShapeCast workgroup-to-subgroup distribution (#171758)

Add special-case handling for ShapeCast when it expands unit dimensions
for a succeeding broadcast op. In this scenario, distribution requires
the source layout to be a slice layout, and the result layout is first
normalized by setting the expanded unit dimensions to 1 before computing
the distributed result shape. In all other cases, ShapeCast is
distributed as usual.

This PR also updates the propagation rule for vectors with expanded unit
dimensions, allowing them to share the same layout as the result of a
broadcast op. This enables correct layout propagation back to the source
of the ShapeCast op, as that layout must ultimately be restored as the
parent layout of the slice layout.

GitOrigin-RevId: 492340aeb1439af6450b1749a3fcda86e967c52e