cmd/8g, cmd/gc: fix warnings on Plan 9

warning: src/cmd/8g/ggen.c:35 non-interruptable temporary
warning: src/cmd/gc/walk.c:656 set and not used: l
warning: src/cmd/gc/walk.c:658 set and not used: l

LGTM=minux.ma
R=golang-codereviews, minux.ma
CC=golang-codereviews
https://golang.org/cl/83660043

src/cmd/8g/ggen.c

@@ -32,7 +32,8 @@ defframe(Prog *ptxt)
 	// so that the garbage collector only sees initialized values
 	// when it looks for pointers.
 	p = ptxt;
-	lo = hi = 0;
+	hi = 0;
+	lo = hi;
 	ax = 0;
 	for(l=curfn->dcl; l != nil; l = l->next) {
 		n = l->n;

src/cmd/gc/walk.c

@@ -653,9 +653,7 @@ walkexpr(Node **np, NodeList **init)
 		r = n->rlist->n;
 		walkexprlistsafe(n->list, init);
 		walkexpr(&r, init);
-		l = n->list->n;
 
-		l = n->list->next->n;
 		ll = ascompatet(n->op, n->list, &r->type, 0, init);
 		n = liststmt(concat(list1(r), ll));
 		goto ret;