clean up negate_iife

- remove extra tree scanning phase for `negate_iife`
- `negate_iife` now only deals with the narrowest form, i.e. IIFE sitting directly under `AST_SimpleStatement`
- `booleans`, `conditionals` etc. will now take care the rest via more accurate accounting
- `a(); void b();` => `a(); b();`

fixes #1288
closes #1451

lib/utils.js

@@ -320,3 +320,26 @@ Dictionary.fromObject = function(obj) {
 function HOP(obj, prop) {
     return Object.prototype.hasOwnProperty.call(obj, prop);
 }
+
+// return true if the node at the top of the stack (that means the
+// innermost node in the current output) is lexically the first in
+// a statement.
+function first_in_statement(stack) {
+    var node = stack.parent(-1);
+    for (var i = 0, p; p = stack.parent(i); i++) {
+        if (p instanceof AST_Statement && p.body === node)
+            return true;
+        if ((p instanceof AST_Seq           && p.car === node        ) ||
+            (p instanceof AST_Call          && p.expression === node && !(p instanceof AST_New) ) ||
+            (p instanceof AST_Dot           && p.expression === node ) ||
+            (p instanceof AST_Sub           && p.expression === node ) ||
+            (p instanceof AST_Conditional   && p.condition === node  ) ||
+            (p instanceof AST_Binary        && p.left === node       ) ||
+            (p instanceof AST_UnaryPostfix  && p.expression === node ))
+        {
+            node = p;
+        } else {
+            return false;
+        }
+    }
+}