fix corner case in reduce_vars (#4663)

fixes #4662

lib/compress.js

@@ -864,7 +864,11 @@ merge(Compressor.prototype, {
                     push_ref(d, lhs);
                     if (d.fixed) {
                         lhs.fixed = d.fixed;
-                        lhs.fixed.assigns = [ node ];
+                        if (lhs.fixed.assigns) {
+                            lhs.fixed.assigns.push(node);
+                        } else {
+                            lhs.fixed.assigns = [ node ];
+                        }
                     }
                 } else {
                     lhs.walk(tw);

test/compress/drop-unused.js

@@ -3251,3 +3251,26 @@ issue_4558_2: {
     }
     expect_stdout: "PASS"
 }
+
+issue_4662: {
+    options = {
+        evaluate: true,
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        var a = 0;
+        function f(b, c) {
+            console.log(b, c);
+        }
+        f(++a, a = a, a);
+    }
+    expect: {
+        var a = 0;
+        (function(b, c) {
+            console.log(b, c);
+        })(++a, a = 1);
+    }
+    expect_stdout: "1 1"
+}