[opt] minor compression ratio improvement #2983
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Following @terrelln's diagnosis #2980,
this is a small follow up
which enforces a minimum cost, of 1 bit per literal, in the cost evaluation of the optimal parser stage,
which is consistent with the hypothesis of a later huffman compression stage.
As can be guessed, this minimum cost enforcement has generally no impact,
since in the vast majority of cases, no literal is dominant.
As explained in #2980, this change is rather meant to take care of some special corner cases.
Nonetheless, I could find 2 files from public corpus which the new policy results in a small compression difference,
favorable in both circumstances :
The benefits also extend to some of the regression tests.