[mlir][linalg] Update vectorization logic for linalg.unpack (#149156)

This PR makes sure that we don't generate unnecessary `tensor.empty`
when vectorizing `linalg.unpack`.

To better visualize the changes implemented here, consider this IR:
```mlir
func.func @example(
  %source: tensor<8x4x16x16xf32>,
  %dest: tensor<64x127xf32>) -> tensor<64x127xf32> {

    %res = linalg.unpack %source
      outer_dims_perm = [1, 0]
      inner_dims_pos = [0, 1]
      inner_tiles = [16, 16]
    into %dest : tensor<8x4x16x16xf32> -> tensor<64x127xf32>

    return %res : tensor<64x127xf32>
 }
```

Below is the output after vectorization, BEFORE and AFTER this PR.

BEFORE (note `tensor.empty` and the fact that `%arg1` is not used):
```mlir
  func.func @example(%arg0: tensor<8x4x16x16xf32>, %arg1: tensor<64x127xf32>) -> tensor<64x127xf32> {
    %cst = arith.constant 0.000000e+00 : f32
    %c0 = arith.constant 0 : index
    %0 = vector.transfer_read %arg0[%c0, %c0, %c0, %c0], %cst {in_bounds = [true, true, true, true]} : tensor<8x4x16x16xf32>, vector<8x4x16x16xf32>
    %1 = vector.transpose %0, [1, 2, 0, 3] : vector<8x4x16x16xf32> to vector<4x16x8x16xf32>
    %2 = vector.shape_cast %1 : vector<4x16x8x16xf32> to vector<64x128xf32>
    %3 = tensor.empty() : tensor<64x127xf32>
    %c0_0 = arith.constant 0 : index
    %4 = vector.transfer_write %2, %3[%c0_0, %c0_0] {in_bounds = [true, false]} : vector<64x128xf32>, tensor<64x127xf32>
    return %4 : tensor<64x127xf32>
  }
```

AFTER (note that `%arg1` is correctly used):
```mlir
  func.func @example(%arg0: tensor<8x4x16x16xf32>, %arg1: tensor<64x127xf32>) -> tensor<64x127xf32> {
    %cst = arith.constant 0.000000e+00 : f32
    %c0 = arith.constant 0 : index
    %0 = vector.transfer_read %arg0[%c0, %c0, %c0, %c0], %cst {in_bounds = [true, true, true, true]} : tensor<8x4x16x16xf32>, vector<8x4x16x16xf32>
    %1 = vector.transpose %0, [1, 2, 0, 3] : vector<8x4x16x16xf32> to vector<4x16x8x16xf32>
    %2 = vector.shape_cast %1 : vector<4x16x8x16xf32> to vector<64x128xf32>
    %c0_0 = arith.constant 0 : index
    %3 = vector.transfer_write %2, %arg1[%c0_0, %c0_0] {in_bounds = [true, false]} : vector<64x128xf32>, tensor<64x127xf32>
    return %3 : tensor<64x127xf32>
  }
```

0 files changed, 0 insertions, 0 deletions