fix invalid transform on const (#1919)

- preserve (re)assignment to `const` for runtime error
- suppress `cascade` on `const`, as runtime behaviour is ill-defined

test/compress/sequences.js

@@ -710,3 +710,27 @@ issue_27: {
         })(jQuery);
     }
 }
+
+reassign_const: {
+    options = {
+        cascade: true,
+        sequences: true,
+        side_effects: true,
+    }
+    input: {
+        function f() {
+            const a = 1;
+            a++;
+            return a;
+        }
+        console.log(f());
+    }
+    expect: {
+        function f() {
+            const a = 1;
+            return a++, a;
+        }
+        console.log(f());
+    }
+    expect_stdout: true
+}