Rotate variable ordering for `mirror-diagonal` by 45 degrees

[Steffan Sølvsten]

Marijn and Randy suggested to rotate the variable ordering by 45 degrees to improve performance. For example, the top-left corner would look as follows.

1 2 5
3 4 7
6 8 9

[Steffan Sølvsten]

As such, this is already the variable ordering in question. In hindsight, this note of mine probably is due to miscommunication.