Optimize partitionNotNullPositions

Split one complicated loop inside partitionNotNullPositions
into two simple ones.
before
Benchmark                                   (channelCount)  (enableCompression)  (nullRate)  (partitionCount)  (positionCount)  (type)  Mode  Cnt    Score    Error  Units
BenchmarkPartitionedOutputOperator.addPage               1                false           0                16             8192  BIGINT  avgt   10  642.430 ± 24.573  ms/op

after
Benchmark                                   (channelCount)  (enableCompression)  (nullRate)  (partitionCount)  (positionCount)  (type)  Mode  Cnt    Score   Error  Units
BenchmarkPartitionedOutputOperator.addPage               1                false           0                16             8192  BIGINT  avgt   10  484.876 ± 4.875  ms/op

core/trino-main/src/main/java/io/trino/operator/output/PagePartitioner.java

@@ -434,9 +434,15 @@ private IntArrayList[] partitionNullablePositions(Page page, int position, IntAr
 
     private IntArrayList[] partitionNotNullPositions(Page page, int startingPosition, IntArrayList[] partitionPositions, IntUnaryOperator partitionFunction)
     {
-        for (int position = startingPosition; position < page.getPositionCount(); position++) {
+        int positionCount = page.getPositionCount();
+        int[] partitionPerPosition = new int[positionCount];
+        for (int position = startingPosition; position < positionCount; position++) {
             int partition = partitionFunction.applyAsInt(position);
-            partitionPositions[partition].add(position);
+            partitionPerPosition[position] = partition;
+        }
+
+        for (int position = startingPosition; position < positionCount; position++) {
+            partitionPositions[partitionPerPosition[position]].add(position);
         }
 
         return partitionPositions;