fix corner case in reduce_vars (#5121)

fixes #5120

test/compress/exponentiation.js

@@ -99,8 +99,8 @@ issue_4664: {
     expect: {
         (function f() {
             new function(a) {
-                console.log(typeof f, 1073741824, typeof this);
-            }(A = 0);
+                console.log(typeof f, a, typeof this);
+            }((A = 0, 2 ** 30));
         })();
     }
     expect_stdout: "function 1073741824 object"

test/compress/functions.js

@@ -6600,3 +6600,32 @@ shorter_without_void: {
         "baz",
     ]
 }
+
+issue_5120: {
+    options = {
+        functions: true,
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        var a = function f() {
+            function g() {
+                f || g();
+            }
+            g();
+            return f.valueOf();
+        };
+        console.log(a() === a ? "PASS" : "FAIL");
+    }
+    expect: {
+        function a() {
+            (function g() {
+                a || g();
+            })();
+            return a.valueOf();
+        }
+        console.log(a() === a ? "PASS" : "FAIL");
+    }
+    expect_stdout: "PASS"
+}