Check permutation map for convolution operations

When computing the (multi input / multi output) kernel matrix for a convolution operator, it is advantageous to permute the rows of the matrix to reduce the number of non-zero diagonals. This in turn reduces the number of diagonals that have to be extracted to compute the MatVec.

The `h` and `w` were interchanged. This is a problem if the filter is non square.

> I'm also noticing an error in flattening the final output in the ISL string.

_Originally posted by @asraa in [#2941](https://github.com/google/heir/pull/2941/changes#r3236751174)_