Preserve program completion value with enable_iife enabled

Fix #640

lib/compress.js

@@ -838,6 +838,14 @@ merge(Compressor.prototype, {
                 return false;
             }
 
+            function is_completion_statement(node) {
+                if (compressor.parent()) {
+                    return false;
+                }
+
+                return statements[statements.length - 1].body === node;
+            }
+
             statements.forEach(function(stat){
                 if (stat instanceof AST_SimpleStatement) {
                     stat.body = (function transform(thing) {
@@ -846,6 +854,8 @@ merge(Compressor.prototype, {
                                 return node;
                             }
                             if (is_iife_call(node)) {
+                                if (is_completion_statement(node)) return node;
+
                                 return make_node(AST_UnaryPrefix, node, {
                                     operator: "!",
                                     expression: node

test/compress/negate-iife.js

@@ -6,7 +6,7 @@ negate_iife_1: {
         (function(){ stuff() })();
     }
     expect: {
-        !function(){ stuff() }();
+        (function(){ stuff() })();
     }
 }
 
@@ -156,7 +156,7 @@ issue_1254_negate_iife_true: {
             };
         })()();
     }
-    expect_exact: '!function(){return function(){console.log("test")}}()();'
+    expect_exact: '(function(){return function(){console.log("test")}})()();'
 }
 
 issue_1254_negate_iife_nested: {
@@ -170,5 +170,17 @@ issue_1254_negate_iife_nested: {
             };
         })()()()()();
     }
-    expect_exact: '!function(){return function(){console.log("test")}}()()()()();'
+    expect_exact: '(function(){return function(){console.log("test")}})()()()()();'
+}
+
+preserve_completion_value: {
+    options = {
+        negate_iife: true,
+    }
+    input: {
+        (function () { return true; })();
+        (function () { return true; })();
+        (function () { return true; })();
+    }
+    expect_exact: '!function(){return true}();!function(){return true}();(function(){return true})();'
 }