Simplify relational witness invariants

[sim642]

Simplifications

1. Use unary minus instead of subtraction from 0: 0 - x >= 0 → -x >= 0.
2. Move constant to other side instead of always comparing with 0: -10 + x >= 0 → x >= 10.
3. Don't add variables with coefficient 0. Apparently they are also iterated over.

TODO

• Avoid negation on both sides: -x >= -5 → 5 >= x.
• Avoid initial unary minus if possible: -x + y >= 0 → y - x >= 0.
• Convert dual octagon inequalities into one equality: x >= 0 && -x >= 0 → x == 0.
  • Use for one var filtering.