Align immutable.{MapN,HashMap} key/value handling with 2.13.x

Fix these properties:

```
    checkAllValuesUpdated(gen, "++ (identical key)")(_.++(Seq((Value(1,1), Value(101,2)))).filter(_._1.id == 1))

    checkAllValuesUpdated(gen, "++ (identical key)")(_.++(Seq((Value(1,1), Value(101,2)))).filter(_._1.id == 1))
```

Unskip immutable.HashMap

Author: retronym

library/src/scala/collection/immutable/HashMap.scala

@@ -366,7 +366,7 @@ object HashMap extends ImmutableMapFactory[HashMap] with BitOperations.Int {
         if (key == this.key) {
           if (merger eq null) {
             if (this.value.asInstanceOf[AnyRef] eq value.asInstanceOf[AnyRef]) this
-            else new HashMap1(key, hash, value, kv)
+            else new HashMap1(this.key, hash, value, null)
           } else if (
             (key.asInstanceOf[AnyRef] eq this.key.asInstanceOf[AnyRef]) &&
               (value.asInstanceOf[AnyRef] eq this.value.asInstanceOf[AnyRef]) &&
@@ -556,7 +556,6 @@ object HashMap extends ImmutableMapFactory[HashMap] with BitOperations.Int {
     private[HashMap] var elems0: Array[HashMap[A, B @uV]],
     private[HashMap] var size0: Int
   ) extends HashMap[A, B @uV] {
-
     @inline private[collection] final def bitmap: Int = bitmap0
     @inline private[collection] final def elems: Array[HashMap[A, B @uV]] = elems0
 

library/src/scala/collection/immutable/Map.scala

@@ -621,12 +621,15 @@ object Map extends ImmutableMapFactory[Map] {
       } else if (elems.size < 4) {
         elems = elems + elem
       } else {
-        // assert(elems.size == 4)
-        elems.get(elem._1) match {
-          case Some(x) if x == elem._2 => ()
-          case _ =>
-          convertToHashMapBuilder()
-          hashMapBuilder += elem
+        val key = elem._1
+        val newValue = elem._2
+        elems.getOrElse(key, Sentinel) match {
+          case Sentinel =>
+            convertToHashMapBuilder()
+            hashMapBuilder += elem
+          case existingValue =>
+            if (existingValue.asInstanceOf[AnyRef] ne newValue.asInstanceOf[AnyRef])
+              elems = elems + elem
         }
       }
       this
@@ -646,6 +649,7 @@ object Map extends ImmutableMapFactory[Map] {
       this
     }
   }
+  private val Sentinel = new Object
 }
 
 /** Explicit instantiation of the `Map` trait to reduce class file size in subclasses. */