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) {