SF patch #707257: Improve code generation

Adds a single function to improve generated bytecode.  Has a single line
attachment point, so it is completely de-coupled from both the compiler
and ceval.c.

Makes three simple transforms that do not require a basic block analysis
or re-ordering of code.  Gives improved timings on pystone, pybench,
and any code using either "while 1" or "x,y=y,x".