cmd/internal/gc: emit branches in -g mode

The -g mode is a debugging mode that prints instructions
as they are constructed. Gbranch was just missing the print.

Change-Id: I3fb45fd9bd3996ed96df5be903b9fd6bd97148b0
Reviewed-on: https://go-review.googlesource.com/9827Reviewed-by: Rick Hudson <rlh@golang.org>

src/cmd/internal/gc/gsubr.go

@@ -90,6 +90,10 @@ func Gbranch(as int, t *Type, likely int) *obj.Prog {
 		p.From.Offset = int64(obj.Bool2int(likely > 0))
 	}
 
+	if Debug['g'] != 0 {
+		fmt.Printf("%v\n", p)
+	}
+
 	return p
 }