ZSTD_ldm_countBackwardsMatch
size_t matchLength = ZSTD_ldm_countBackwardsMatch(pIn, pAnchor, pMatch, pMatchBase);
matchLength += ZSTD_ldm_countBackwardsMatch(pIn - matchLength, pAnchor, pExtDictEnd, pExtDictStart);
ZSTD_ldm_countBackwardsMatch(split, anchor, pMatch, lowPrefixPtr);