if x and y are real, x**y is real indeed and real and imaginary parts
do not need any further expansion.
Fix 188fce4: Pow.is_real is True if base>=0 and exp>=0
Fix cse-related failure in test_issue_3022
Move new Pow.is_real tests to the right place
Merge pull request #2788 from skirpichev/pow-real-fix

Pow simplification: if x and y are >=0 then x**y is real