fix undefined confusion with local variable

lib/compress.js

@@ -1054,7 +1054,7 @@ merge(Compressor.prototype, {
                         stat.value = cons_seq(stat.value);
                     }
                     else if (stat instanceof AST_Exit) {
-                        stat.value = cons_seq(make_node(AST_Undefined, stat));
+                        stat.value = cons_seq(make_node(AST_Undefined, stat).transform(compressor));
                     }
                     else if (stat instanceof AST_Switch) {
                         stat.expression = cons_seq(stat.expression);
@@ -2912,7 +2912,7 @@ merge(Compressor.prototype, {
                 if (name instanceof AST_SymbolRef
                     && name.name == "console"
                     && name.undeclared()) {
-                    return make_node(AST_Undefined, self).transform(compressor);
+                    return make_node(AST_Undefined, self).optimize(compressor);
                 }
             }
         }

test/compress/sequences.js

@@ -440,3 +440,29 @@ func_def_5: {
     }
     expect_stdout: "true"
 }
+
+issue_1758: {
+    options = {
+        sequences: true,
+        side_effects: true,
+    }
+    input: {
+        console.log(function(c) {
+            var undefined = 42;
+            return function() {
+                c--;
+                c--, c.toString();
+                return;
+            }();
+        }());
+    }
+    expect:{
+        console.log(function(c) {
+            var undefined = 42;
+            return function() {
+                return c--, c--, c.toString(), void 0;
+            }();
+        }());
+    }
+    expect_stdout: "undefined"
+}