[mlir][Vector] Fix bug in vector xfer op flattening transformation (#81964)

It looks like the affine map generated to compute the indices of the
collapsed dimensions used the wrong dim size. For indices `[idx0][idx1]`
we computed the collapsed index as `idx0*size0 + idx1` instead of
`idx0*size1 + idx1`. This led to correctness issues in convolution tests
when enabling this transformation internally.