fix corner case collapse_vars (#5569)

fixes #5568

test/compress/collapse_vars.js

@@ -9979,3 +9979,25 @@ issue_5396: {
     }
     expect_stdout: "PASS"
 }
+
+issue_5568: {
+    options = {
+        collapse_vars: true,
+        evaluate: true,
+        reduce_vars: true,
+        toplevel: true,
+        unused: true,
+    }
+    input: {
+        A = "FAIL";
+        var a = (A = "PASS", !1);
+        for (var b in a);
+        console.log(A);
+    }
+    expect: {
+        A = "FAIL";
+        for (var b in !(A = "PASS"));
+        console.log(A);
+    }
+    expect_stdout: "PASS"
+}

test/reduce.js

@@ -460,6 +460,16 @@ module.exports = function reduce_test(testcase, minify_options, reduce_options)
                 return to_statement(node.definitions[0].value);
             }
         }
+        else if (node instanceof U.AST_VarDef) {
+            if (node.value) {
+                node.start._permute++;
+                CHANGED = true;
+                return new U.AST_VarDef({
+                    name: node.name,
+                    start: {},
+                });
+            }
+        }
 
         if (in_list) {
             // drop switch branches