Issue-20619 early return existing value. (dotnet#46481)

* Issue-20619 early return existing value.

* issue-20619 Fix comment: Reuse check inside of SetItem; Also use customer-supplied value comparer for TryUpdate().

* issue-20619 Fix tab-space issue.

* issue-20619 Fix comment: Revert: use customer-supplied value comparer for TryUpdate().

src/libraries/System.Collections.Immutable/src/System/Collections/Immutable/ImmutableInterlocked.cs

@@ -354,6 +354,10 @@ public static TValue AddOrUpdate<TKey, TValue>(ref ImmutableDictionary<TKey, TVa
                 }
 
                 var updatedCollection = priorCollection.SetItem(key, newValue);
+                if (object.ReferenceEquals(priorCollection, updatedCollection))
+                {
+                    return oldValue;
+                }
                 var interlockedResult = Interlocked.CompareExchange(ref location, updatedCollection, priorCollection);
                 successful = object.ReferenceEquals(priorCollection, interlockedResult);
                 priorCollection = interlockedResult; // we already have a volatile read that we can reuse for the next loop
@@ -397,6 +401,10 @@ public static TValue AddOrUpdate<TKey, TValue>(ref ImmutableDictionary<TKey, TVa
                 }
 
                 var updatedCollection = priorCollection.SetItem(key, newValue);
+                if (object.ReferenceEquals(priorCollection, updatedCollection))
+                {
+                    return oldValue;
+                }
                 var interlockedResult = Interlocked.CompareExchange(ref location, updatedCollection, priorCollection);
                 successful = object.ReferenceEquals(priorCollection, interlockedResult);
                 priorCollection = interlockedResult; // we already have a volatile read that we can reuse for the next loop