cmd/compile: fix bug in oconv

Introduced by https://go-review.googlesource.com/#/c/28331/ .

Change-Id: Id75aed6410f06b302d5347f6ca6a2e19c61f6fb6
Reviewed-on: https://go-review.googlesource.com/28779
Reviewed-by: Matthew Dempsky <mdempsky@google.com>

src/cmd/compile/internal/gc/fmt.go

@@ -244,7 +244,7 @@ func (o Op) oconv(s fmt.State) {
 		return
 	}
 
-	fmt.Sprintf("O-%d", o)
+	fmt.Fprintf(s, "O-%d", o)
 }
 
 var classnames = []string{