IRTools dynamos + Zygote type issues

This particular issue is really centered around `Zygote`, but I'm guessing that `IRTools` is part of the cause.

In my probabilistic programming framework, I have _learnable_ parameters:
```julia
function learnable_hypers()
l = learnable(:l, Float64[4.0, 4.0])
m = learnable(:m, 10.0)
q = rand(:q, Normal(l[1], 1.0 + exp(m)))
return q
end
```
for which gradients can be computed. But I'm having drastically different behavior depending on type annotations for parameters with `Array` values.

In particular, the above works correctly with my pullbacks defined [here](https://github.com/femtomc/Jaynes.jl/blob/master/src/contexts/backpropagate.jl). However, if I change the type annotation:

```julia
function learnable_hypers()
l = learnable(:l, Any[4.0, 4.0])
m = learnable(:m, 10.0)
q = rand(:q, Normal(l[1], 1.0 + exp(m)))
return q
end
```

I get an array mutation error:
```
ERROR: LoadError: Mutating arrays is not supported
Stacktrace:
[1] error(::String) at ./error.jl:33
[2] (::Zygote.var"#1048#1049")(::Nothing) at /home/mccoy/.julia/packages/Zygote/YeCEW/src/lib/array.jl:61
[3] (::Zygote.var"#2775#back#1050"{Zygote.var"#1048#1049"})(::Nothing) at /home/mccoy/.julia/packages/ZygoteRules/6nssF/src/adjoint.jl:49
[4] hvcat_fill at ./abstractarray.jl:1707 [inlined]
[5] (::typeof(∂(λ)))(::Nothing) at /home/mccoy/.julia/packages/Zygote/YeCEW/src/compiler/interface2.jl:0
[6] typed_hvcat at ./abstractarray.jl:1729 [inlined]
[7] (::typeof(∂(λ)))(::Nothing) at /home/mccoy/.julia/packages/Zygote/YeCEW/src/compiler/interface2.jl:0
[8] learnable_hypers at /home/mccoy/.julia/dev/Jaynes/scratch/learnable.jl:8 [inlined]
[9] (::typeof(∂(λ)))(::Float64) at /home/mccoy/.julia/packages/Zygote/YeCEW/src/compiler/interface2.jl:0
[10] #174 at /home/mccoy/.julia/packages/Zygote/YeCEW/src/lib/lib.jl:182 [inlined]
[11] #347#back at /home/mccoy/.julia/packages/ZygoteRules/6nssF/src/adjoint.jl:49 [inlined]
[12] #84 at /home/mccoy/.julia/dev/Jaynes/src/contexts/backpropagate.jl:152 [inlined]
[13] (::typeof(∂(λ)))(::Tuple{Float64,Float64}) at /home/mccoy/.julia/packages/Zygote/YeCEW/src/compiler/interface2.jl:0
[14] (::Zygote.var"#36#37"{typeof(∂(λ))})(::Tuple{Float64,Float64}) at /home/mccoy/.julia/packages/Zygote/YeCEW/src/compiler/interface.jl:46
[15] accumulate_parameter_gradients!(::Main.Learnable.Jaynes.Gradients, ::Main.Learnable.Jaynes.BlackBoxCallSite{Main.Learnable.Jaynes.HierarchicalTrace,Tuple{},Float64}, ::Float64, ::Float64) at /home/mccoy/.julia/dev/Jaynes/src/contexts/backpropagate.jl:157
[16] get_parameter_gradients at /home/mccoy/.julia/dev/Jaynes/src/contexts/backpropagate.jl:204 [inlined]
[17] get_parameter_gradients(::Main.Learnable.Jaynes.BlackBoxCallSite{Main.Learnable.Jaynes.HierarchicalTrace,Tuple{},Float64}, ::Float64) at /home/mccoy/.julia/dev/Jaynes/src/contexts/backpropagate.jl:203
```

Wierdly enough, I run into the same issue if I type annotate multi-dimensional arrays

```julia
function learnable_hypers()
l = learnable(:l, Float64[4.0 4.0; 4.0 4.0])
m = learnable(:m, 10.0)
q = rand(:q, Normal(l[1], 1.0 + exp(m)))
return q
end
```

So 1-dimensional `Float64` arrays seem to work, but everything else will not go. Path forward might be to analyze what's going on with `Cthulhu`.