crypto/types: optimize compact bit array one count (#9216)

By using bits.OnesCount8 instead of bitwise operations.

Benchmark result on Apple M1:

name                     old time/op    new time/op    delta
NumTrueBitsBefore/new-8    88.5ns ± 1%     9.1ns ± 0%  -89.68%  (p=0.000 n=10+10)

name                     old alloc/op   new alloc/op   delta
NumTrueBitsBefore/new-8     0.00B          0.00B          ~     (all equal)

name                     old allocs/op  new allocs/op  delta
NumTrueBitsBefore/new-8      0.00           0.00          ~     (all equal)

Fixes #9125

crypto/types/compact_bit_array.go

@@ -6,6 +6,7 @@ import (
 	"errors"
 	"fmt"
 	"math"
+	"math/bits"
 	"regexp"
 	"strings"
 )
@@ -85,23 +86,18 @@ func (bA *CompactBitArray) SetIndex(i int, v bool) bool {
 // given index. e.g. if bA = _XX__XX, NumOfTrueBitsBefore(4) = 2, since
 // there are two bits set to true before index 4.
 func (bA *CompactBitArray) NumTrueBitsBefore(index int) int {
-	numTrueValues := 0
+	onesCount := 0
 	max := bA.Count()
 	if index > max {
 		index = max
 	}
 	// below we iterate over the bytes then over bits (in low endian) and count bits set to 1
-	var i = 0
 	for elem := 0; ; elem++ {
-		for b := 7; b >= 0; b-- {
-			if i >= index {
-				return numTrueValues
-			}
-			i++
-			if (bA.Elems[elem]>>b)&1 == 1 {
-				numTrueValues++
-			}
+		if elem*8+7 >= index {
+			onesCount += bits.OnesCount8(bA.Elems[elem] >> (7 - (index % 8) + 1))
+			return onesCount
 		}
+		onesCount += bits.OnesCount8(bA.Elems[elem])
 	}
 }