ldstopt: Use exchange() instead of set_irn_in() when a Sync has only a single operand left.

ir/opt/ldstopt.c

@@ -2243,6 +2243,9 @@ again:;
 				new_in[new_arity++] = in;
 		}
 
+		if (new_arity == 1)
+			exchange(sync, new_in[0]);
+		else
 			set_irn_in(sync, new_arity, new_in);
 	}
 }