Propagate conditional field type of mutable struct

[Roger-luo]

I already checked fn.kwargs is not nothing but it seems JET still does a union split on fn.kwargs and reporting false positive on method call of nothing here

https://github.com/Roger-luo/Expronicon.jl/blob/main/src/codegen.jl#L65

reported via

JET.report_package("Expronicon")

related results

┌ @ /Users/roger/Code/Julia/Expronicon/src/codegen.jl:67 fn.kwargs[1]
│ no matching method found `getindex(::Nothing, ::Int64)` (1/2 union split): ((fn::Expronicon.JLFunction).kwargs::Union{Nothing, Vector{Any}})[1]
└────────────────────────────────────────────────────────
┌ @ /Users/roger/Code/Julia/Expronicon/src/codegen.jl:68 fn.kwargs[1]
│ no matching method found `getindex(::Nothing, ::Int64)` (1/2 union split): ((fn::Expronicon.JLFunction).kwargs::Union{Nothing, Vector{Any}})[1]
└────────────────────────────────────────────────────────

[aviatesk]

If you define JLFunction as immutable struct, JET should no longer report the issue. Fixing this for mutable case requires escape analysis.