process code with implicit return statement (#1522)

Bookmarklet for instance implicitedly assumes a "completion value" without using `return`.
The `expression` option now supports such use cases.
Optimisations on IIFEs also enhanced.

fixes #354
fixes #543
fixes #625
fixes #628
fixes #640
closes #1293

test/compress/sequences.js

@@ -248,6 +248,6 @@ iife: {
     }
     expect: {
         x = 42, function a() {}(), function b() {}(), function c() {}(),
-        function d() {}(), function e() {}(), function f() {}(), function g() {}()
+        function d() {}(), function e() {}(), function f() {}(), function g() {}();
     }
 }