Let EncodingAttr track the op type

[bjacob]

Currently we only data-tile matmuls. When we start data-tiling convs, we will need to track in the encoding the type of op using the encoded tensors. This doesn't map 1:1 to linalg named ops. This could be a new enumeration attribute, EncodingOpType, that could for now have a single value MATMUL (used for all of the flavors of matmuls that we currently handle, including batch matmuls, vecmat, matvec etc). Then when we start data-tiling convs, there could be a new value CONV.

Note: in the past we used to have such an enum, EncodingUser. I don't think that was a great name.