pprof: replace bits = bits + "..." to bits += "..." where bits is a string.

Change-Id: Ic77ebbdf2670b7fdf2c381cd1ba768624b07e57c
Reviewed-on: https://go-review.googlesource.com/c/141998
Run-TryBot: Iskander Sharipov <iskander.sharipov@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/pprof/internal/profile/profile.go

@@ -415,16 +415,16 @@ func (p *Profile) String() string {
 	for _, m := range p.Mapping {
 		bits := ""
 		if m.HasFunctions {
-			bits = bits + "[FN]"
+			bits += "[FN]"
 		}
 		if m.HasFilenames {
-			bits = bits + "[FL]"
+			bits += "[FL]"
 		}
 		if m.HasLineNumbers {
-			bits = bits + "[LN]"
+			bits += "[LN]"
 		}
 		if m.HasInlineFrames {
-			bits = bits + "[IN]"
+			bits += "[IN]"
 		}
 		ss = append(ss, fmt.Sprintf("%d: %#x/%#x/%#x %s %s %s",
 			m.ID,