Zygote fails with Char datatypes

[staticfloat]

With the latest master of Zygote and IRTools:

struct Foo
    x
end

function (f::Foo)(x, mode)
    if mode == 'a'
        return x*f.x
    else
        return x/f.x
    end
end

xf, back = Zygote.forward(f -> f(1, 'a'), Foo(2.0))
back(1.0)

This results in:

ERROR: Compiling Tuple{getfield(Main, Symbol("##13#14")),Foo}: MethodError: no method matching exprtype(::Core.Compiler.IRCode, ::Char)
Closest candidates are:
  exprtype(::Core.Compiler.IRCode, ::Expr) at /Users/sabae/.julia/packages/Zygote/UFYYV/src/tools/ir.jl:64
  exprtype(::Core.Compiler.IRCode, ::QuoteNode) at /Users/sabae/.julia/packages/Zygote/UFYYV/src/tools/ir.jl:61
  exprtype(::Core.Compiler.IRCode, ::GlobalRef) at /Users/sabae/.julia/packages/Zygote/UFYYV/src/tools/ir.jl:60
  ...
Stacktrace:
 [1] _broadcast_getindex_evalf at ./broadcast.jl:582 [inlined]
 [2] _broadcast_getindex at ./broadcast.jl:555 [inlined]
 [3] getindex at ./broadcast.jl:515 [inlined]
 [4] copyto_nonleaf!(::Array{DataType,1}, ::Base.Broadcast.Broadcasted{Base.Broadcast.DefaultArrayStyle{1},Tuple{Base.OneTo{Int64}},typeof(Zygote.exprtype),Tuple{Base.RefValue{Core.Compiler.IRCode},Base.Broadcast.Extruded{Array{Any,1},Tuple{Bool},Tuple{Int64}}}}, ::Base.OneTo{Int64}, ::Int64, ::Int64) at ./broadcast.jl:931
 [5] copy at ./broadcast.jl:794 [inlined]
 [6] materialize(::Base.Broadcast.Broadcasted{Base.Broadcast.DefaultArrayStyle{1},Nothing,typeof(Zygote.exprtype),Tuple{Base.RefValue{Core.Compiler.IRCode},Array{Any,1}}}) at ./broadcast.jl:756
 [7] record!(::Core.Compiler.IRCode) at /Users/sabae/.julia/packages/Zygote/UFYYV/src/compiler/reverse.jl:151
 [8] #Primal#39(::Nothing, ::Type, ::Core.Compiler.IRCode) at /Users/sabae/.julia/packages/Zygote/UFYYV/src/compiler/reverse.jl:196
 [9] Type at ./none:0 [inlined]
 [10] #Adjoint#65 at /Users/sabae/.julia/packages/Zygote/UFYYV/src/compiler/reverse.jl:394 [inlined]
 [11] (::getfield(Core, Symbol("#kw#Type")))(::NamedTuple{(:varargs,),Tuple{Nothing}}, ::Type{Zygote.Adjoint}, ::Core.Compiler.IRCode) at ./none:0
 [12] _lookup_grad(::Type) at /Users/sabae/.julia/packages/Zygote/UFYYV/src/compiler/emit.jl:125
 [13] #s79#919 at /Users/sabae/.julia/packages/Zygote/UFYYV/src/compiler/interface2.jl:17 [inlined]
 [14] #s79#919(::Any, ::Any, ::Any) at ./none:0
 [15] (::Core.GeneratedFunctionStub)(::Any, ::Vararg{Any,N} where N) at ./boot.jl:506
 [16] (::getfield(Zygote, Symbol("##66#67")){Zygote.Pullback{Tuple{getfield(Main, Symbol("##13#14")),Foo},Tuple{getfield(Main, Symbol("##13#14"))}}})(::Float64) at /Users/sabae/.julia/packages/Zygote/UFYYV/src/compiler/interface.jl:38
 [17] top-level scope at none:0