Optimize arithmetic comparisons

[lukaszcz]

After merging

• PR Numeric, ordering and equality traits #2433

numeric comparisons have become inefficient even with optimization, because they now go through Ord.cmp.

We're left with terms like

case if = high n then EQ else if < high n then LT else GT of {
            LT := true;
            EQ := false;
            GT := false
          }

We should optimize the above to high < n.