forward_diff_no_inf can't handle throw_undef_if_not

[oxinabox]

Ran into this while trying to reproduce another bug.
The following code errors:

using Diffractor
# this is needed as transform! is *always* called on Arguments regardless of what visit_custom says
identity_transform!(ir, ssa::Core.SSAValue, order) = ir[ssa]
function identity_transform!(ir, arg::Core.Argument, order)
    return Core.Compiler.insert_node!(ir, Core.SSAValue(1), Core.Compiler.NewInstruction(Expr(:call, Diffractor.ZeroBundle{1}, arg), Any))
end

function phi_run(x::Float64, b1::Bool, b2::Bool)
    if b1
        a = 0.
    elseif b2
        a = z
    end
    return x - a
end

input_ir = first(only(Base.code_ircode(phi_run, Tuple{Float64, Bool, Bool})))
Diffractor.forward_diff_no_inf!(copy(input_ir), Core.SSAValue.(1:length(input_ir.stmts)) .=> 1; transform! = identity_transform!)

I think the fix is easy, add the case to forward_visit about what to do for this kind of node.
Which i think is return nothing