[Mosaic GPU] Do not shortcut the transform computation for `memref.cast`
As it turns out the result of `in_transforms`/`out_transforms` cannot be returned directly as it has the wrong type (one extra wrapping with `ir.ArrayAttr`. I will remove this for now and consider refactoring the whole file in the future to make the types clearer.
PiperOrigin-RevId: 755844780