v0.3.2 broke Meta.code.edges

[oxinabox]

Actual Behavior

this is how it is on 0.3.2 and master

julia> using IRTools  # @0.3.2 (also current master)

julia> m = IRTools.meta(Tuple{typeof(+), Int, Int})
Metadata for +(x::T, y::T) where T<:Union{Int128, Int16, Int32, Int64, Int8, UInt128, UInt16, UInt32, UInt64, UInt8} in Base at int.jl:53

julia> typeof(m.code.edges)
Nothing

Expected Behavior

This is how it was in 0.3.1

julia> using IRTools  # @0.3.1

julia> m = IRTools.meta(Tuple{typeof(+), Int, Int})
Metadata for +(x::T, y::T) where T<:Union{Int128, Int16, Int32, Int64, Int8, UInt128, UInt16, UInt32, UInt64, UInt8} in Base at int.jl:53

julia> typeof(m.code.edges)
Array{Core.MethodInstance,1}

[MikeInnes]

Caused by 7e077bf. For flexibility we just set the edges later. I'm sure we can do that in the chainrules PR. We should also figure out the right stable API for this and get that PR to use it.