FFC generates the wrong code for negative powers with tensor representation

[Martin Sandve Alnæs]

Tensor representation generates wrong code for this expression:

u = Constant(triangle)
#q = (1 / u)**2 # Fails as it should (no division handler)
q = u**(-2) # Produces wrong code!
M = q*dx

It should detect the problem and fail as the first case.

[Martin Sandve Alnæs]

Resolved i 537af164ae.