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/functions.js

@@ -35,9 +35,9 @@ iifes_returning_constants_keep_fargs_true: {
         console.log("okay");
         console.log(123);
         console.log(void 0);
-        console.log(function(x,y,z){return 2}(1,2,3));
-        console.log(function(x,y){return 6}(2,3));
-        console.log(function(x, y){return 6}(2,3,a(),b()));
+        console.log(2);
+        console.log(6);
+        console.log((a(), b(), 6));
     }
 }
 
@@ -71,6 +71,6 @@ iifes_returning_constants_keep_fargs_false: {
         console.log(void 0);
         console.log(2);
         console.log(6);
-        console.log(function(){return 6}(a(),b()));
+        console.log((a(), b(), 6));
     }
 }