[dev.cc] cmd/gc: tweak default fatal in ordersafeexpr for c2go

c2go was putting a fallthrough after the fatal call.
Changed c2go to know that fatal doesn't return,
but then there is a missing return at the end of
the translated Go function.
Move code around a little to make C and Go agree.

Change-Id: Icef3d55ccdde0709c02dd0c2b78826f6da33a146
Reviewed-on: https://go-review.googlesource.com/5170Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

src/cmd/gc/order.c

@@ -144,9 +144,6 @@ ordersafeexpr(Node *n, Order *order)
 	Node *l, *r, *a;
 	
 	switch(n->op) {
-	default:
-		fatal("ordersafeexpr %O", n->op);
-
 	case ONAME:
 	case OLITERAL:
 		return n;
@@ -191,6 +188,9 @@ ordersafeexpr(Node *n, Order *order)
 		typecheck(&a, Erv);
 		return a;
 	}
+
+	fatal("ordersafeexpr %O", n->op);
+	return nil; // not reached
 }		
 
 // Istemp reports whether n is a temporary variable.