[mlir][transform] Add elementwise criteria to `match.structured.body` (#79626)

As far as I am aware, there is no simple way to match on elementwise
ops. I propose to add an `elementwise` criteria to the
`match.structured.body` op. Although my only hesitation is that
elementwise is not only determined by the body, but also the indexing
maps. So if others find this too awkward, I can implement a separate
match op instead.