Factorization failure for certain conditionals

While probably quite a corner case, I managed to stumble into that...
```
import ufl
from ffcx.codegeneration import jit

elmt = ufl.FiniteElement("CG", ufl.triangle, 1)

u1 = ufl.Coefficient(elmt)
u2 = ufl.Coefficient(elmt)
ctol = ufl.Constant(ufl.triangle)

def cond(tol):
return ufl.conditional(u2 < tol, u1, u2) * ufl.dx

# works
jit.compile_forms([ufl.derivative(cond(0), (u1, u2))])

# works
jit.compile_forms([ufl.derivative(cond(ctol), u1)])

# works
jit.compile_forms([ufl.derivative(cond(ctol), u2)])

# fails
jit.compile_forms([ufl.derivative(cond(ctol), (u1, u2))])

```