fix corner case in unused (#3665)

fixes #3664

lib/compress.js

@@ -3913,7 +3913,7 @@ merge(Compressor.prototype, {
                     if (node instanceof AST_Assign) {
                         if (!in_use || node.left === sym && def.id in fixed_ids && fixed_ids[def.id] !== node) {
                             value = get_rhs(node);
-                            if (node.write_only) {
+                            if (node.write_only === true) {
                                 value = value.drop_side_effect_free(compressor) || make_node(AST_Number, node, {
                                     value: 0
                                 });

test/compress/drop-unused.js

@@ -2335,3 +2335,23 @@ function_parameter_ie8: {
     }
     expect_stdout: "PASS"
 }
+
+issue_3664: {
+    options = {
+        pure_getters: "strict",
+        unused: true,
+    }
+    input: {
+        console.log(function() {
+            var a, b = (a = (a = [ b && console.log("FAIL") ]).p = 0, 0);
+            return "PASS";
+        }());
+    }
+    expect: {
+        console.log(function() {
+            var b = ([ b && console.log("FAIL") ].p = 0, 0);
+            return "PASS";
+        }());
+    }
+    expect_stdout: "PASS"
+}