chore: fix implicit-reducible diamond in digraphs (#38291)

The following fails on master, works after the PR:
```lean
example : (Digraph.instPartialOrder.toLE : LE (Digraph V)) = 
    Digraph.instLE := by
  with_reducible_and_instances rfl
```

Co-authored-by: sgouezel <sebastien.gouezel@univ-rennes1.fr>

Author: sgouezel

Mathlib/Combinatorics/Digraph/Basic.lean

@@ -167,11 +167,9 @@ theorem iSup_adj {f : ι → Digraph V} : (⨆ i, f i).Adj a b ↔ ∃ i, (f i).
 @[simp]
 theorem iInf_adj {f : ι → Digraph V} : (⨅ i, f i).Adj a b ↔ (∀ i, (f i).Adj a b) := by simp [iInf]
 
-instance : PartialOrder (Digraph V) where
-  __ := PartialOrder.lift _ adj_injective
-  le G H := ∀ ⦃a b⦄, G.Adj a b → H.Adj a b
+instance : PartialOrder (Digraph V) := fast_instance% PartialOrder.lift _ adj_injective
 
-instance distribLattice : DistribLattice (Digraph V) :=
+instance distribLattice : DistribLattice (Digraph V) := fast_instance%
   adj_injective.distribLattice _ .rfl .rfl (fun _ _ ↦ rfl) fun _ _ ↦ rfl
 
 instance completeAtomicBooleanAlgebra : CompleteAtomicBooleanAlgebra (Digraph V) where