Pipelined Implementation of ZSTD_fast (~+5% Speed) #2749

Amusingly, it seems to be a non-trivial performance hit to add in final searches or even hash table insertions during cleanup. So let's not. It seems to not make any meaningful difference in compression ratio.

… Speed) Unrolling the loop to handle 2 positions in each iteration allows us to reduce the frequency of some operations that don't need to happen at every position. One such operation is the step calculation, which is a very rough heuristic anyways. It's fine if we do this a position later. The other operation is the repcode check. But since the repcode check already tries expanding back one position, we're really not missing much of importance by only trying it every other position. This commit also slightly reorders some operations.

This removes the old `ZSTD_compressBlock_fast_generic()` and renames the new `ZSTD_compressBlock_fast_generic_pipelined()` to replace it. This is functionally a no-op.

It's a bit strange, because this is hitting the dictionary special case where the dictionary is contiguous with the input and still runs in the single- segment path. We should probably change that to hit the `extDict` path instead?

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Pipelined Implementation of ZSTD_fast (~+5% Speed) #2749

Pipelined Implementation of ZSTD_fast (~+5% Speed) #2749

Commits on Sep 1, 2021