Pow-factor usage unification

[ETatuzova]

There are few different places in placeholder where pow_factor construction is used.
result = \sum_i=0 ^n \alpha^i c_i

But sometimes it has form:
result = \sum_i=0 ^n \alpha^{n-i} c_i

Due to our plans to create an efficient pow_factor component we need to provide similar pow_factor usage to increase pow_factor efficiency.