s2: Better should not search repeat value

[klauspost]

Remove repeat checks in better mode since they are no often than not stealing better matches.

Instead check if matches are repeats when emitting. Similar performance, decompression prefers this.

Before/After:

nyc-taxi:
Compressing... 3325605752 -> 954776589 [28.71%]; 492ms, 6445.5MB/s
Compressing... 3325605752 -> 935022950 [28.12%]; 489ms, 6485.8MB/s
Decompressing. 954776589 -> 3325605752 [348.31%]; 2.434s, 1302.9MB/s
Decompressing. 935022950 -> 3325605752 [355.67%]; 2.297s, 1381.0MB/s

enwik9:
Compressing... 1000000000 -> 426854233 [42.69%]; 213ms, 4477.3MB/s
Compressing... 1000000000 -> 426242923 [42.62%]; 219ms, 4354.7MB/s
Decompressing. 426242923 -> 1000000000 [234.61%]; 996ms, 957.0MB/s
Decompressing. 426854233 -> 1000000000 [234.27%]; 1.009s, 945.2MB/s