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

Change-Id: I840262eefff1073a452a243b9e31f61b53362fcf
Reviewed-on: https://go-review.googlesource.com/c/go/+/428360
Run-TryBot: Robert Griesemer <[email protected]>
Reviewed-by: Robert Griesemer <[email protected]>
Reviewed-by: Ian Lance Taylor <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
Run-TryBot: Ian Lance Taylor <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>

Author: cuiweixie

src/cmd/compile/internal/ssagen/nowb.go

@@ -5,8 +5,8 @@
 package ssagen
 
 import (
-	"bytes"
 	"fmt"
+	"strings"
 
 	"cmd/compile/internal/base"
 	"cmd/compile/internal/ir"
@@ -179,7 +179,7 @@ func (c *nowritebarrierrecChecker) check() {
 
 		// Check fn.
 		if fn.WBPos.IsKnown() {
-			var err bytes.Buffer
+			var err strings.Builder
 			call := funcs[fn]
 			for call.target != nil {
 				fmt.Fprintf(&err, "\n\t%v: called by %v", base.FmtPos(call.lineno), call.target.Nname)

src/cmd/compile/internal/ssagen/ssa.go

@@ -7249,7 +7249,7 @@ func genssa(f *ssa.Func, pp *objw.Progs) {
 		}
 	}
 	if f.HTMLWriter != nil { // spew to ssa.html
-		var buf bytes.Buffer
+		var buf strings.Builder
 		buf.WriteString("<code>")
 		buf.WriteString("<dl class=\"ssa-gen\">")
 		filename := ""