Convert power of 2 $pow cells to shift

[georgerennie]

Feature Description

Yosys understandably doesn't bother trying to synthesize $pow cells as there is no sensible way of lowering them in a general way, but for the specific case of the first argument being a power of 2, it can be converted to a left shift in a standard optimization.

Although ideally users should try not to use ** in the synthesised part of their code, there are some real world examples where it is being used in this manner, and it would be good to be able to support them.

This probably should be a peepopt or opt_expr rule.