Optimisation -> Optimization.
Improved warning.
Create Not if possible.

This fixes endless local optimization in 400.perlbench.
Added local optimization: (a - b) ==/!= 0 => a ==/!= b.

Also improved local optimization for Eor.
Added local optimization: (a ^ b) == a => b == 0.
Reduce code duplication in transform_bitop_chain().