Improve prefix sum in Lucene99HnswVectorsReader

Lucene99HnswVectorsReader computes prefix sums by doing:

        currentNeighborsBuffer[0] = dataIn.readVInt();
        for (int i = 1; i < arcCount; i++) {
          currentNeighborsBuffer[i] = currentNeighborsBuffer[i - 1] + dataIn.readVInt();
        }

However, the investigation on #14979 suggests that a faster way of writing this loop would be

        int sum = 0;
        for (int i = 0; i < arcCount; i++) {
          sum += dataIn.readVInt();
          currentNeighborsBuffer[i] = sum;
        }