Simplify degenerate ifelse cases. Fix Symbolics.jl#170

[Omar-Elrefaei]

The change was tested with the issue's reproducible sample

julia> @variables x
julia> ∂ₓ = Differential(x)
julia> expr = expand_derivatives(∂ₓ(∂ₓ(max(1,x))))
IfElse.ifelse(1 > x, 0, 0)
julia> simplify(expr)

and now we yield a 0 instead of IfElse.ifelse(1 > x, 0, 0)

[YingboMa]

Could you add a test?