Compute complete_ag that contains irreducible alias information

```julia
julia> @variables t x(t) y(t) z(t);

julia> D = Differential(t)
(::Differential) (generic function with 2 methods)

julia> eqs = [x - y + 2 ~ 0; D(y) ~ 0; z - D(x) ~ 0]; @nAmed sys = ODESystem(eqs, t);

julia> alias_elimination(sys);
┌ Info:
└   vv => aa = Differential(t)(y(t)) => 0
┌ Info:
└   vv => aa = z(t) => Differential(t)(x(t))
```

Author: YingboMa

src/systems/alias_elimination.jl

@@ -39,7 +39,7 @@ alias_elimination(sys) = alias_elimination!(TearingState(sys; quick_cancel = tru
 function alias_elimination!(state::TearingState)
     sys = state.sys
     complete!(state.structure)
-    ag, mm, updated_diff_vars = alias_eliminate_graph!(state)
+    ag, complete_ag, mm, updated_diff_vars = alias_eliminate_graph!(state)
     isempty(ag) && return sys
 
     fullvars = state.fullvars
@@ -543,6 +543,7 @@ function alias_eliminate_graph!(graph, var_to_diff, mm_orig::SparseMatrixCLIL)
     #
     nvars = ndsts(graph)
     ag = AliasGraph(nvars)
+    complete_ag = AliasGraph(nvars)
     mm, echelon_mm = simple_aliases!(ag, graph, var_to_diff, mm_orig)
 
     # Step 3: Handle differentiated variables
@@ -702,6 +703,7 @@ function alias_eliminate_graph!(graph, var_to_diff, mm_orig::SparseMatrixCLIL)
             while (iv = diff_to_var[v]) in zero_vars_set
                 v = iv
             end
+            complete_ag[v] = 0
             if diff_to_var[v] === nothing # `v` is reducible
                 dag[v] = 0
             end
@@ -729,6 +731,13 @@ function alias_eliminate_graph!(graph, var_to_diff, mm_orig::SparseMatrixCLIL)
     # Step 4: Merge dag and ag
     removed_aliases = BitSet()
     merged_ag = AliasGraph(nvars)
+    for (v, (c, a)) in dag
+        complete_ag[v] = c => a
+    end
+    for (v, (c, a)) in ag
+        (processed[v] || (!iszero(a) && processed[a])) && continue
+        complete_ag[v] = c => a
+    end
     for (v, (c, a)) in dag
         # D(x) ~ D(y) cannot be removed if x and y are not aliases
         if v != a && !iszero(a)
@@ -789,7 +798,7 @@ function alias_eliminate_graph!(graph, var_to_diff, mm_orig::SparseMatrixCLIL)
         update_graph_neighbors!(graph, ag)
     end
 
-    return ag, mm, updated_diff_vars
+    return ag, complete_ag, mm, updated_diff_vars
 end
 
 function update_graph_neighbors!(graph, ag)