[mlir][linalg] Use vector.shuffle to flatten conv filter (#75038)

Updates the vectorisation of 1D depthwise convolution when flattening
the channel dimension (introduced in #71918). In particular - how the
convolution filter is "flattened". ATM, the vectoriser will use
`vector.shape_cast`:

```mlir
  %b_filter = vector.broadcast %filter : vector<4xf32> to vector<3x2x4xf32>
  %sc_filter = vector.shape_cast %b_filter : vector<3x2x4xf32> to vector<3x8xf32>
```

This lowering is not ideal - `vector.shape_cast` can be convenient when
it's folded away, but that's not happening in this case. Instead, this
patch updates the vectoriser to use `vector.shuffle` (the overall result
is identical):

```mlir
  %sh_filter = vector.shuffle %filter, %filter
      [0, 1, 2, 3, 0, 1, 2, 3] : vector<4xf32>, vector<4xf32>
  %b_filter = vector.broadcast %sh_filter : vector<8xf32> to vector<3x8xf32>
```