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Reduces the cost of "not" to almost zero.

Add a new opcode, LIST_APPEND, and apply it to the code generation for
list comprehensions.  Reduces the per-loop overhead by about a third.

From SF patch #852334.

The additional code complexity and new NOP opcode were not worth it.

* Can now test for basic blocks.
* Optimize inverted comparisions.
* Optimize unary_not followed by a conditional jump.
* Added a new opcode, NOP, to keep code size constant.
* Applied NOP to previous transformations where appropriate.

Note, the NOP would not be necessary if other functions were
added to re-target jump addresses and update the co_lnotab mapping.
That would yield slightly faster and cleaner bytecode at the
expense of optimizer simplicity and of keeping it decoupled
from the line-numbering structure.

should be generated automatically