Move support for negate_iife in the compressor, rather than code generator

(the code generator doesn't maintain enough context to know whether
the return value is important or discarded)

Fixes #272

lib/compress.js

@@ -66,6 +66,7 @@ function Compressor(options, false_by_default) {
         join_vars     : !false_by_default,
         cascade       : !false_by_default,
         side_effects  : !false_by_default,
+        negate_iife   : !false_by_default,
         screw_ie8     : false,
 
         warnings      : true,
@@ -214,6 +215,11 @@ merge(Compressor.prototype, {
                 statements = join_consecutive_vars(statements, compressor);
             }
         } while (CHANGED);
+
+        if (compressor.option("negate_iife")) {
+            negate_iifes(statements, compressor);
+        }
+
         return statements;
 
         function eliminate_spurious_blocks(statements) {
@@ -497,6 +503,40 @@ merge(Compressor.prototype, {
             }, []);
         };
 
+        function negate_iifes(statements, compressor) {
+            statements.forEach(function(stat){
+                if (stat instanceof AST_SimpleStatement) {
+                    stat.body = (function transform(thing) {
+                        return thing.transform(new TreeTransformer(function(node){
+                            if (node instanceof AST_Call && node.expression instanceof AST_Function) {
+                                return make_node(AST_UnaryPrefix, node, {
+                                    operator: "!",
+                                    expression: node
+                                });
+                            }
+                            else if (node instanceof AST_Call) {
+                                node.expression = transform(node.expression);
+                            }
+                            else if (node instanceof AST_Seq) {
+                                node.car = transform(node.car);
+                            }
+                            else if (node instanceof AST_Conditional) {
+                                var expr = transform(node.condition);
+                                if (expr !== node.condition) {
+                                    // it has been negated, reverse
+                                    node.condition = expr;
+                                    var tmp = node.consequent;
+                                    node.consequent = node.alternative;
+                                    node.alternative = tmp;
+                                }
+                            }
+                            return node;
+                        }));
+                    })(stat.body);
+                }
+            });
+        };
+
     };
 
     function extract_declarations_from_unreachable_code(compressor, stat, target) {

lib/output.js

@@ -61,7 +61,6 @@ function OutputStream(options) {
         comments      : false,
         preserve_line : false,
         screw_ie8     : false,
-        negate_iife   : !(options && options.beautify),
     }, true);
 
     var indentation = 0;
@@ -351,21 +350,17 @@ function OutputStream(options) {
 
     AST_Node.DEFMETHOD("print", function(stream, force_parens){
         var self = this, generator = self._codegen;
-        stream.push_node(self);
-        var needs_parens = self.needs_parens(stream);
-        var fc = self instanceof AST_Function && stream.option("negate_iife");
-        if (force_parens || (needs_parens && !fc)) {
-            stream.with_parens(function(){
-                self.add_comments(stream);
-                self.add_source_map(stream);
-                generator(self, stream);
-            });
-        } else {
+        function doit() {
             self.add_comments(stream);
-            if (needs_parens && fc) stream.print("!");
             self.add_source_map(stream);
             generator(self, stream);
         }
+        stream.push_node(self);
+        if (force_parens || self.needs_parens(stream)) {
+            stream.with_parens(doit);
+        } else {
+            doit();
+        }
         stream.pop_node();
     });