drop side_effects-free return values (#2965)

lib/compress.js

@@ -3644,7 +3644,15 @@ merge(Compressor.prototype, {
                 if (this.expression instanceof AST_Function
                     && (!this.expression.name || !this.expression.name.definition().references.length)) {
                     var node = this.clone();
-                    node.expression.process_expression(false, compressor);
+                    var exp = node.expression;
+                    exp.process_expression(false, compressor);
+                    exp.walk(new TreeWalker(function(node) {
+                        if (node instanceof AST_Return && node.value) {
+                            node.value = node.value.drop_side_effect_free(compressor);
+                            return true;
+                        }
+                        if (node instanceof AST_Scope && node !== exp) return true;
+                    }));
                     return node;
                 }
                 return this;