[LDM] Speed optimization on repetitive data

[LDM] Speed optimization on repetitive data

LDM does especially poorly on repetitive data when that data's hash happens
to have `(hash & stopMask) == 0`. Either because the `stopMask == 0` or
random chance. Optimize this case by skipping over repetitive patterns.
The detection is very simplistic, but should catch most of the offending
cases.

```
head -c 1G /dev/zero | perf stat -- ./zstd -1 -o /dev/null -v --zstd=ldmHashRateLog=1 --long
      21.187881087 seconds time elapsed

head -c 1G /dev/zero | perf stat -- ./zstd -1 -o /dev/null -v --zstd=ldmHashRateLog=1 --long
       1.149707921 seconds time elapsed

```