few more optimizations:

- do multiple passes in tighten_body if it was changed
- transform if (foo) return x; return y; ==> return foo?x:y
- don't optimize !0 as true (use best_of after evaluation of constant expr)

With hoist_vars off we now beat UglifyJS v1 on jQuery-1.8.1

test/compress/conditionals.js

@@ -90,7 +90,7 @@ ifs_4: {
 
 ifs_5: {
     options = {
-        conditionals: true
+        if_return: true
     };
     input: {
         function f() {