Refactor MpscLinkedQueue.poll

Handle empty queue first, then share most of the
implementation for non-empty scenarios (spin and
non-spin).

Author: olivergillespie

src/main/java/io/reactivex/rxjava3/internal/queue/MpscLinkedQueue.java

@@ -87,23 +87,19 @@ public boolean offer(final T e) {
     public T poll() {
         LinkedQueueNode<T> currConsumerNode = lpConsumerNode(); // don't load twice, it's alright
         LinkedQueueNode<T> nextNode = currConsumerNode.lvNext();
-        if (nextNode != null) {
-            // we have to null out the value because we are going to hang on to the node
-            final T nextValue = nextNode.getAndNullValue();
-            spConsumerNode(nextNode);
-            return nextValue;
+        final T nextValue;
+        if (nextNode == null && currConsumerNode == lvProducerNode()) {
+            return null;
         }
-        else if (currConsumerNode != lvProducerNode()) {
+        if (nextNode == null) {
             // spin, we are no longer wait free
             while ((nextNode = currConsumerNode.lvNext()) == null) { } // NOPMD
             // got the next node...
-
-            // we have to null out the value because we are going to hang on to the node
-            final T nextValue = nextNode.getAndNullValue();
-            spConsumerNode(nextNode);
-            return nextValue;
         }
-        return null;
+        // we have to null out the value because we are going to hang on to the node
+        nextValue = nextNode.getAndNullValue();
+        spConsumerNode(nextNode);
+        return nextValue;
     }
 
     @Override