deeper prefetching pipeline for decompressSequencesLong

deeper prefetching pipeline for decompressSequencesLong

pipeline increased from 4 to 8 slots.
This change substantially improves decompression speed when there are long distance offsets.
example with enwik9 compressed at level 22 :
gcc-9 : 947 -> 1039 MB/s
clang-10: 884 -> 946 MB/s

I also checked the "cold dictionary" scenario,
and found a smaller benefit, around ~2%
(measurements are more noisy for this scenario).