improve decompression speed of long variant by ~+5%

improve decompression speed of long variant by ~+5%

changed strategy,
now unconditionally prefetch the first 2 cache lines,
instead of cache lines corresponding to the first and last bytes of the match.

This better corresponds to cpu expectation,
which should auto-prefetch following cachelines on detecting the sequential nature of the read.

This is globally positive, by +5%,
though exact gains depend on compiler (from -2% to +15%).
The only negative counter-example is gcc-9.