🥯 all: relax b.Loop usage

The documentation of b.Loop mentions that the compiler has a special case for it, and it "must be written in exactly" the form of "for b.Loop() { ... }".

Turns out it's actually not that restrictive. As of Go 1.24, the check is at https://github.com/golang/go/blob/master/src/cmd/compile/internal/inline/interleaved/interleaved.go. It has no restrictions on the init and post part of the for loop.

Given this knowledge, this commit moves scattered "var i int" and "i++" into the for statement.

domainset/matcher_domain_test.go

@@ -131,12 +131,10 @@ func TestDomainMapMatcher(t *testing.T) {
 
 func benchmarkDomainMatcher(b *testing.B, count int, name string, m Matcher) {
 	b.Run(fmt.Sprintf("%d/%s/Hit", count, name), func(b *testing.B) {
-		var i int
-		for b.Loop() {
+		for i := 0; b.Loop(); i++ {
 			if !m.Match(testDomains[i%count]) {
 				b.Fatal("unexpected miss")
 			}
-			i++
 		}
 	})
 	b.Run(fmt.Sprintf("%d/%s/Miss", count, name), func(b *testing.B) {

domainset/matcher_suffix_test.go

@@ -127,12 +127,10 @@ func TestSuffixTrieMatcher(t *testing.T) {
 
 func benchmarkSuffixMatcher(b *testing.B, count int, name string, m Matcher) {
 	b.Run(fmt.Sprintf("%d/%s/Hit", count, name), func(b *testing.B) {
-		var i int
-		for b.Loop() {
+		for i := 0; b.Loop(); i++ {
 			if !m.Match(testSuffixes[i%count]) {
 				b.Fatal("unexpected miss")
 			}
-			i++
 		}
 	})
 	b.Run(fmt.Sprintf("%d/%s/Miss/Short", count, name), func(b *testing.B) {