[mlir][linalg] Fix invalid IR in `FoldInsertPadIntoFill` (#74418)

`FoldInsertPadIntoFill` used to generate an invalid
`tensor.insert_slice` op:
```
error: expected type to be 'tensor<?x?x?xf32>' or a rank-reduced version. (size mismatch)
```

This commit fixes tests such as
`mlir/test/Dialect/Linalg/canonicalize.mlir` when verifying the IR after
each pattern application (#74270).