[issue2181] optimize out local variables at end of function

[Neal Norwitz]

Neal Norwitz added the comment:

I forgot to mention that if another loop was added to PyCode_Optimize
that kept track of the # of times each local variable was
LOAD_FAST/STORE_FAST/DELETE_FAST and that the count was 2, we could
perform a similar optimization without requiring the return.

Bonus points for other cases like if it was the last use inside a list
comprehension or the variable is otherwise unaccessible.

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