Zygote breaks @isdefined

[Pangoraw]

julia> Zygote.gradient(1.0) do x # [email protected]
           if @isdefined(x)
               2x
           else
               3x
           end
       end
(3.0,)

This is problematic because

@info "hello" x

uses @isdefined for single variables like this and logging more complex expressions is now supported because of #1474. The problem might require fixes in IRTools as well which does not have a great support for this (it replaces undefs with IRTools.Undef()).