`call` backedges sometimes dropped in presence of identical `invoke` edges

[topolarity]

Issue to track #57079

PkgA:

module PkgA
    struct DummyStruct{T} end

    bar(@nospecialize(::DummyStruct)) = error("unimplemented")
    function foo(@nospecialize(x::DummyStruct))
        if @noinline rand(Bool)
            # Add an 'invoke' edge from 'foo' to 'bar'
            Core.invoke(bar, Tuple{DummyStruct}, x)
        else
            # ... and also an identical 'call' edge
            @noinline bar(x)
        end
    end
end

PkgB:

module PkgB
    import PkgA: DummyStruct, foo, bar

    # This should invalidate 'foo'
    bar(::DummyStruct{Int}) = 15
    
    # The new 'foo' that is precompiled needs to add the
    # correct backedges to 'bar(::DummyStruct)'.
    #
    # These are the same backedges from before, but this
    # tests that they survive de-serialization by an
    # external package.
    precompile(foo, (DummyStruct,))
end

When loading:

julia> using PkgA, PkgB

julia> m = methods(PkgA.bar, (PkgA.DummyStruct,))[end]
bar(::PkgA.DummyStruct)
     @ PkgA ~/repos/julia/PkgA/src/PkgA.jl:5

julia> m.specializations.backedges
2-element Vector{Any}:
 Tuple{typeof(PkgA.bar), PkgA.DummyStruct}
 MethodInstance for PkgA.foo(::PkgA.DummyStruct)

On 1.10 / 1.11, we're missing a call backedge. The backedge list should be:

julia> m.specializations.backedges
3-element Vector{Any}:
 Tuple{typeof(PkgA.bar), PkgA.DummyStruct}
 MethodInstance for PkgA.foo(::PkgA.DummyStruct)
 MethodInstance for PkgA.foo(::PkgA.DummyStruct)