Function loopTile

Tiles a perfect 2-deep loop nest by tileSize.

Transforms:

do i = lo_i, hi_i
  do j = lo_j, hi_j
    body
  end do
end do
Copy

Into:

do ii = lo_i, hi_i, TILE
  do jj = lo_j, hi_j, TILE
    do i = ii, MIN(ii + TILE - 1, hi_i)
      do j = jj, MIN(jj + TILE - 1, hi_j)
        body
      end do
    end do
  end do
end do
Copy

Returns unchanged [outer] if either loop is not a unit-step range loop.