[mlir][Linalg] Evolve named ops to use assembly form and support linalg on tensors.

This revision allows representing a reduction at the level of linalg on tensors for named ops. When a structured op has a reduction and returns tensor(s), new conventions are added and documented.

As an illustration, the syntax for a `linalg.matmul` writing into a buffer is:

```
  linalg.matmul ins(%a, %b : memref<?x?xf32>, tensor<?x?xf32>)
               outs(%c : memref<?x?xf32>)
```

, whereas the syntax for a `linalg.matmul` returning a new tensor is:

```
  %d = linalg.matmul ins(%a, %b : tensor<?x?xf32>, memref<?x?xf32>)
                    init(%c : memref<?x?xf32>)
                      -> tensor<?x?xf32>
```

Other parts of linalg will be extended accordingly to allow mixed buffer/tensor semantics in the presence of reductions.