huff0: Check for zeros earlier in Scratch.countSimple (#704)

v == 0 implies v <= m, so we can skip a CMOV in that case. Benchmarks
show a tiny gain on most inputs and a large gain on the few with very
small alphabets:

```
name                                      old speed      new speed       delta
Compress1XReuseNone/digits-8               432MB/s ± 1%    434MB/s ± 1%     ~     (p=0.023 n=14+35)
Compress1XReuseNone/gettysburg-8           241MB/s ± 1%    242MB/s ± 1%   +0.58%  (p=0.000 n=14+34)
Compress1XReuseNone/twain-8                352MB/s ± 1%    354MB/s ± 1%   +0.62%  (p=0.000 n=14+29)
Compress1XReuseNone/low-ent.10k-8          460MB/s ± 2%    464MB/s ± 1%   +0.92%  (p=0.000 n=14+30)
Compress1XReuseNone/superlow-ent-10k-8     304MB/s ± 1%    305MB/s ± 1%     ~     (p=0.040 n=15+33)
Compress1XReuseNone/crash2-8              10.6MB/s ± 1%   10.9MB/s ± 1%   +3.28%  (p=0.000 n=15+33)
Compress1XReuseNone/endzerobits-8         13.1MB/s ± 4%   15.6MB/s ± 4%  +19.15%  (p=0.000 n=15+34)
Compress1XReuseNone/endnonzero-8          7.26MB/s ± 2%   7.36MB/s ± 1%   +1.44%  (p=0.000 n=15+32)
Compress1XReuseNone/case1-8               13.3MB/s ± 1%   13.4MB/s ± 1%   +0.71%  (p=0.000 n=15+35)
Compress1XReuseNone/case2-8               11.1MB/s ± 1%   11.2MB/s ± 1%   +0.57%  (p=0.005 n=14+34)
Compress1XReuseNone/case3-8               11.6MB/s ± 1%   11.8MB/s ± 2%   +1.22%  (p=0.000 n=15+34)
Compress1XReuseNone/pngdata.001-8          298MB/s ± 1%    300MB/s ± 1%     ~     (p=0.019 n=15+35)
Compress1XReuseNone/normcount2-8          32.3MB/s ± 1%   32.7MB/s ± 1%   +1.14%  (p=0.000 n=15+34)
Compress1XReuseAllow/digits-8              433MB/s ± 1%    436MB/s ± 1%   +0.68%  (p=0.000 n=15+33)
Compress1XReuseAllow/gettysburg-8          269MB/s ± 1%    272MB/s ± 1%   +1.26%  (p=0.000 n=15+32)
Compress1XReuseAllow/twain-8               361MB/s ± 1%    363MB/s ± 1%   +0.47%  (p=0.002 n=15+34)
Compress1XReuseAllow/low-ent.10k-8         465MB/s ± 1%    466MB/s ± 1%     ~     (p=0.155 n=15+34)
Compress1XReuseAllow/superlow-ent-10k-8    303MB/s ± 1%    304MB/s ± 1%     ~     (p=0.074 n=13+35)
Compress1XReuseAllow/crash2-8             14.0MB/s ± 1%   14.7MB/s ± 1%   +5.34%  (p=0.000 n=15+35)
Compress1XReuseAllow/endzerobits-8        13.9MB/s ± 4%   16.9MB/s ± 1%  +21.23%  (p=0.000 n=14+32)
Compress1XReuseAllow/endnonzero-8         9.92MB/s ± 3%  10.97MB/s ± 2%  +10.61%  (p=0.000 n=15+31)
Compress1XReuseAllow/case1-8              15.8MB/s ± 1%   16.0MB/s ± 2%   +1.42%  (p=0.000 n=15+33)
Compress1XReuseAllow/case2-8              13.3MB/s ± 1%   13.5MB/s ± 1%   +1.52%  (p=0.000 n=14+32)
Compress1XReuseAllow/case3-8              14.1MB/s ± 1%   14.2MB/s ± 2%   +1.21%  (p=0.001 n=15+34)
Compress1XReuseAllow/pngdata.001-8         301MB/s ± 1%    301MB/s ± 1%     ~     (p=0.948 n=15+34)
Compress1XReuseAllow/normcount2-8         40.6MB/s ± 1%   41.4MB/s ± 1%   +2.03%  (p=0.000 n=15+31)
Compress1XReusePrefer/digits-8             435MB/s ± 1%    437MB/s ± 2%     ~     (p=0.046 n=15+34)
Compress1XReusePrefer/gettysburg-8         420MB/s ± 2%    425MB/s ± 2%   +1.25%  (p=0.000 n=15+34)
Compress1XReusePrefer/twain-8              363MB/s ± 1%    364MB/s ± 1%   +0.43%  (p=0.003 n=15+34)
Compress1XReusePrefer/low-ent.10k-8        468MB/s ± 1%    469MB/s ± 1%     ~     (p=0.029 n=15+34)
Compress1XReusePrefer/superlow-ent-10k-8   308MB/s ± 1%    310MB/s ± 1%   +0.66%  (p=0.000 n=15+34)
Compress1XReusePrefer/crash2-8            50.7MB/s ± 2%   61.5MB/s ± 2%  +21.25%  (p=0.000 n=14+33)
Compress1XReusePrefer/endzerobits-8       19.0MB/s ± 1%   24.4MB/s ± 2%  +28.68%  (p=0.000 n=13+30)
Compress1XReusePrefer/endnonzero-8        25.7MB/s ± 1%   33.1MB/s ± 1%  +28.60%  (p=0.000 n=14+32)
Compress1XReusePrefer/case1-8              129MB/s ± 6%    149MB/s ± 6%  +15.55%  (p=0.000 n=15+35)
Compress1XReusePrefer/case2-8              124MB/s ± 1%    143MB/s ± 1%  +14.84%  (p=0.000 n=14+30)
Compress1XReusePrefer/case3-8              132MB/s ± 1%    154MB/s ± 1%  +16.91%  (p=0.000 n=15+34)
Compress1XReusePrefer/pngdata.001-8        312MB/s ± 1%    312MB/s ± 1%     ~     (p=0.936 n=14+28)
Compress1XReusePrefer/normcount2-8         192MB/s ± 1%    207MB/s ± 1%   +7.98%  (p=0.000 n=14+34)
Compress1XSizes/digits-100-8              59.2MB/s ± 1%   60.7MB/s ± 1%   +2.56%  (p=0.000 n=15+34)
Compress1XSizes/digits-200-8               104MB/s ± 1%    107MB/s ± 1%   +2.69%  (p=0.000 n=15+32)
Compress1XSizes/digits-500-8               195MB/s ± 1%    199MB/s ± 1%   +2.04%  (p=0.000 n=14+35)
Compress1XSizes/digits-1000-8              278MB/s ± 1%    283MB/s ± 1%   +1.55%  (p=0.000 n=14+34)
Compress1XSizes/digits-5000-8              416MB/s ± 1%    420MB/s ± 1%   +0.82%  (p=0.000 n=15+31)
Compress1XSizes/digits-10000-8             440MB/s ± 1%    442MB/s ± 1%   +0.50%  (p=0.004 n=15+34)
Compress1XSizes/digits-50000-8             434MB/s ± 1%    436MB/s ± 1%   +0.44%  (p=0.008 n=15+34)
```

huff0/compress.go

@@ -365,29 +365,29 @@ func (s *Scratch) countSimple(in []byte) (max int, reuse bool) {
 	m := uint32(0)
 	if len(s.prevTable) > 0 {
 		for i, v := range s.count[:] {
+			if v == 0 {
+				continue
+			}
 			if v > m {
 				m = v
 			}
-			if v > 0 {
-				s.symbolLen = uint16(i) + 1
-				if i >= len(s.prevTable) {
-					reuse = false
-				} else {
-					if s.prevTable[i].nBits == 0 {
-						reuse = false
-					}
-				}
+			s.symbolLen = uint16(i) + 1
+			if i >= len(s.prevTable) {
+				reuse = false
+			} else if s.prevTable[i].nBits == 0 {
+				reuse = false
 			}
 		}
 		return int(m), reuse
 	}
 	for i, v := range s.count[:] {
+		if v == 0 {
+			continue
+		}
 		if v > m {
 			m = v
 		}
-		if v > 0 {
-			s.symbolLen = uint16(i) + 1
-		}
+		s.symbolLen = uint16(i) + 1
 	}
 	return int(m), false
 }