FFT3 optimization

[xtonik]

Optimization of FFT3 method done in that ways:

1. Different way of computing FFT elements indexes.
2. Reordering commands in various way - this seems to be very unpredictable behavior.
3. Reduced number of plus operation via two intermediate sums.
4. Reduced number of temporary variables.

The way no.3 standing standalone worsened, but with other optimization help well. Performance was improved by 10-20%, the cost for it is worsened readability of the code.