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* Bump `WILDCOPY_OVERLENGTH` to 16 to fix the wildcopy overread. * Optimize `ZSTD_wildcopy()` by removing unnecessary branches and unrolling the loop. * Extract `ZSTD_overlapCopy8()` into its own function. * Add `ZSTD_safecopy()` for `ZSTD_execSequenceEnd()`. It is optimized for single long sequences, since that is the important case that can end up in `ZSTD_execSequenceEnd()`. Without this optimization, decompressing a block with 1 long match goes from 5.7 GB/s to 800 MB/s. * Refactor `ZSTD_execSequenceEnd()`. * Increase the literal copy shortcut to 16. * Add a shortcut for offset >= 16. * Simplify `ZSTD_execSequence()` by pushing more cases into `ZSTD_execSequenceEnd()`. * Delete `ZSTD_execSequenceLong()` since it is exactly the same as `ZSTD_execSequence()`. clang-8 seeds +17.5% on silesia and +21.8% on enwik8. gcc-9 sees +12% on silesia and +15.5% on enwik8. TODO: More detailed measurements, and on more datasets. Crdit to OSS-Fuzz for finding the wildcopy overread.