do not call tocomplex() twice for __pow
do not call tocomplex() twice for __pow

Test Plan: ran it

Reviewed By: kma@fb.com

Subscribers: lesha, jhj

FB internal diff: D1452382

soumith/fblualib,szagoruyko/fblualib,szagoruyko/fblualib,soumith/fblualib,facebook/fblualib,facebook/fblualib,raphaelamorim/fblualib,raphaelamorim/fblualib