Fixed pre/post increment/decrement in assignment operations.

Fixed pre/post increment/decrement in assignment operations.

Previously, the compound assignment operations did not create temporary
index for increments/decrements in the generator. The result was that
the increment/decrement changed the value immediately in place, which
led to incorrect calculations.

The fix is to use a separate temporary index for increments/decrements in
assignment operations.

This closes #271 issue on GitHub.