aot: move `jl_insert_backedges` to Julia side (#56499)
With JuliaLang/julia#56447, the dependency between `jl_insert_backedges`
and method insertion has been eliminated, allowing `jl_insert_backedges`
to be performed after loading. As a result, it is now possible to move
`jl_insert_backedges` to the Julia side.
Currently this commit simply moves the implementation without adding any
new features.
---------
Co-authored-by: Jameson Nash <vtjnash@gmail.com>
