cmd/compile/internal/abt: use strings.Builder

Since go1.17 is now used for bootstrapping.

Change-Id: I5692ff4b38b7d9a279b20662ce5348ff34a2dde6
Reviewed-on: https://go-review.googlesource.com/c/go/+/427134
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Reviewed-by: hopehook <hopehook@golangcn.org>
Run-TryBot: Cuong Manh Le <cuong.manhle.vn@gmail.com>
Reviewed-by: David Chase <drchase@google.com>

src/cmd/compile/internal/abt/avlint32.go

@@ -7,6 +7,7 @@ package abt
 import (
 	"fmt"
 	"strconv"
+	"strings"
 )
 
 const (
@@ -326,39 +327,21 @@ func (t *T) Equals(u *T) bool {
 	return t.root.equals(u.root)
 }
 
-// This doesn't build with go1.4, sigh
-// func (t *T) String() string {
-// 	var b strings.Builder
-// 	first := true
-// 	for it := t.Iterator(); !it.IsEmpty(); {
-// 		k, v := it.Next()
-// 		if first {
-// 			first = false
-// 		} else {
-// 			b.WriteString("; ")
-// 		}
-// 		b.WriteString(strconv.FormatInt(int64(k), 10))
-// 		b.WriteString(":")
-// 		b.WriteString(v.String())
-// 	}
-// 	return b.String()
-// }
-
 func (t *T) String() string {
-	var b string
+	var b strings.Builder
 	first := true
 	for it := t.Iterator(); !it.Done(); {
 		k, v := it.Next()
 		if first {
 			first = false
 		} else {
-			b += ("; ")
+			b.WriteString("; ")
 		}
-		b += (strconv.FormatInt(int64(k), 10))
+		b.WriteString(strconv.FormatInt(int64(k), 10))
-		b += (":")
+		b.WriteString(":")
-		b += fmt.Sprint(v)
+		b.WriteString(fmt.Sprint(v))
 	}
-	return b
+	return b.String()
 }
 
 func (t *node32) equals(u *node32) bool {