fix corner case in reduce_vars

fixes #4581

lib/compress.js

@@ -719,7 +719,7 @@ merge(Compressor.prototype, {
             var fn = this;
             fn.inlined = false;
             var iife = tw.parent();
-            var hit = fn instanceof AST_AsyncFunction;
+            var hit = is_async(fn);
             var aborts = false;
             fn.walk(new TreeWalker(function(node) {
                 if (hit) return aborts = true;

test/compress/awaits.js

@@ -1003,3 +1003,24 @@ issue_4534: {
     expect_stdout: "PASS"
     node_version: ">=8"
 }
+
+issue_4581: {
+    options = {
+        evaluate: true,
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        var a = "PASS";
+        (async () => (A, a = "FAIL"))();
+        console.log(a);
+    }
+    expect: {
+        var a = "PASS";
+        (async () => (A, a = "FAIL"))();
+        console.log(a);
+    }
+    expect_stdout: "PASS"
+    node_version: ">=8"
+}