Reduce RowHash's tag space size by x2 (#3543)

Allocate half the memory for tag space, which means that we get one less slot for an actual tag (needs to be used for next position index).
The results is a slight loss in compression ratio (up to 0.2%) and some regressions/improvements to speed depending on level and sample. In turn, we get to save 16% of the hash table's space (5 bytes per entry instead of 6 bytes per entry).

lib/compress/zstd_compress_internal.h

@@ -226,7 +226,7 @@ struct ZSTD_matchState_t {
     U32 hashLog3;           /* dispatch table for matches of len==3 : larger == faster, more memory */
 
     U32 rowHashLog;                          /* For row-based matchfinder: Hashlog based on nb of rows in the hashTable.*/
-    U16* tagTable;                           /* For row-based matchFinder: A row-based table containing the hashes and head index. */
+    BYTE* tagTable;                          /* For row-based matchFinder: A row-based table containing the hashes and head index. */
     U32 hashCache[ZSTD_ROW_HASH_CACHE_SIZE]; /* For row-based matchFinder: a cache of hashes to improve speed */
 
     U32* hashTable;