zstd: Improve throughput of SpeedBestCompression encoder (#699)

Lifted ofCode and mlCode computations out of match.estBits, so that
method will be inlined into its only caller. Also some changes to
eliminate a branch: the last if block becomes two CMOVs/CSELs on
amd64 and arm64.

```
name                              old speed      new speed       delta
Encoder_EncodeAllSimple/best-8    11.1MB/s ± 1%   16.9MB/s ± 1%  +52.23%  (p=0.000 n=10+10)
Encoder_EncodeAllSimple4K/best-8  8.41MB/s ± 1%  10.95MB/s ± 0%  +30.20%  (p=0.000 n=10+10)

name                              old alloc/op   new alloc/op    delta
Encoder_EncodeAllSimple/best-8       20.0B ± 0%      18.0B ± 0%  -10.00%  (p=0.002 n=8+10)
Encoder_EncodeAllSimple4K/best-8     2.00B ± 0%      2.00B ± 0%     ~     (all equal)
```

zstd/enc_best.go

@@ -38,27 +38,29 @@ type match struct {
 const highScore = 25000
 
 // estBits will estimate output bits from predefined tables.
-func (m *match) estBits(bitsPerByte int32) {
-	mlc := mlCode(uint32(m.length - zstdMinMatch))
-	var ofc uint8
-	if m.rep < 0 {
-		ofc = ofCode(uint32(m.s-m.offset) + 3)
-	} else {
-		ofc = ofCode(uint32(m.rep))
-	}
+func (m *match) estBits(bitsPerByte int32, ofc, mlc uint8) {
 	// Cost, excluding
 	ofTT, mlTT := fsePredefEnc[tableOffsets].ct.symbolTT[ofc], fsePredefEnc[tableMatchLengths].ct.symbolTT[mlc]
 
 	// Add cost of match encoding...
-	m.est = int32(ofTT.outBits + mlTT.outBits)
-	m.est += int32(ofTT.deltaNbBits>>16 + mlTT.deltaNbBits>>16)
+	est := int32(ofTT.outBits + mlTT.outBits)
+	est += int32(ofTT.deltaNbBits>>16 + mlTT.deltaNbBits>>16)
 	// Subtract savings compared to literal encoding...
-	m.est -= (m.length * bitsPerByte) >> 10
-	if m.est > 0 {
+	length := m.length
+	est -= (length * bitsPerByte) >> 10
+	if est > 0 {
 		// Unlikely gain..
-		m.length = 0
-		m.est = highScore
+		est, length = highScore, 0
+	}
+	m.est, m.length = est, length
+}
+
+func (m *match) ofCode() uint8 {
+	offset := uint32(m.rep)
+	if offset < 0 {
+		offset = uint32(m.s-m.offset) + 3
 	}
+	return ofCode(offset)
 }
 
 // bestFastEncoder uses 2 tables, one for short matches (5 bytes) and one for long matches.
@@ -216,7 +218,7 @@ encodeLoop:
 				}
 			}
 			m := match{offset: offset, s: s, length: 4 + e.matchlen(s+4, offset+4, src), rep: rep}
-			m.estBits(bitsPerByte)
+			m.estBits(bitsPerByte, m.ofCode(), mlCode(uint32(m.length-zstdMinMatch)))
 			return m
 		}