[vector][mlir] Restrict vector.shuffle to fixed-width vectors (#88733)

At the moment there is no support for vector.shuffle for scalable
vectors - various hooks/helpers related to `vector.shuffle` simply
ignore the scalable flags (e.g. ` ShuffleOp::inferReturnTypes`).

This is unlikely to change any time soon (vector shuffles are known to
be tricky for scalable vectors), hence this patch restricts
`vector.shuffle` to fixed width vectors.