cmd/9g: use nopout in excise

In addition to duplicating the logic, the old code was
clearing the line number, which led to missing source line
information in the -S output.

Also fix nopout, which was incomplete.

Change-Id: Ic2b596a2f9ec2fe85642ebe125cca8ef38c83085
Reviewed-on: https://go-review.googlesource.com/3512Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Dave Cheney <dave@cheney.net>

src/cmd/9g/gobj.c

@@ -236,5 +236,8 @@ void
 nopout(Prog *p)
 {
 	p->as = ANOP;
+	p->from = zprog.from;
+	p->from3 = zprog.from3;
+	p->reg = zprog.reg;
+	p->to = zprog.to;
 }
-

src/cmd/9g/peep.c

@@ -327,15 +327,12 @@ ret:
 void
 excise(Flow *r)
 {
-	Prog *p, *l;
+	Prog *p;
 
 	p = r->prog;
 	if(debug['P'] && debug['v'])
 		print("%P ===delete===\n", p);
-	l = p->link;
-	*p = zprog;
-	p->as = ANOP;
-	p->link = l;
+	nopout(p);
 	ostats.ndelmov++;
 }