fix corner case in reduce_vars

fixes #3880

lib/compress.js

@@ -797,8 +797,12 @@ merge(Compressor.prototype, {
                     d.recursive_refs++;
                 } else if (value && ref_once(tw, compressor, d)) {
                     d.in_loop = tw.loop_ids[d.id] !== tw.in_loop;
-                    d.single_use = value instanceof AST_Lambda && !value.pinned()
-                        || !d.in_loop && d.scope === this.scope && value.is_constant_expression();
+                    d.single_use = value instanceof AST_Lambda
+                            && !value.pinned()
+                            && (!d.in_loop || tw.parent() instanceof AST_Call)
+                        || !d.in_loop
+                            && d.scope === this.scope
+                            && value.is_constant_expression();
                 } else {
                     d.single_use = false;
                 }

test/compress/reduce_vars.js

@@ -7047,3 +7047,24 @@ issue_3866: {
     }
     expect_stdout: "PASS"
 }
+
+issue_3880: {
+    options = {
+        reduce_funcs: true,
+        reduce_vars: true,
+        unused: true,
+    }
+    input: {
+        (function(a) {
+            while (a.var ^= 1);
+            console.log("PASS");
+        })(function() {});
+    }
+    expect: {
+        (function(a) {
+            while (a.var ^= 1);
+            console.log("PASS");
+        })(function() {});
+    }
+    expect_stdout: "PASS"
+}