[regexp] Fix case-insensitive matching for one-byte subjects.

test/mjsunit/regress/regress-5199.js

Index: test/mjsunit/regress/regress-5199.js
diff --git a/test/message/regress/regress-4829-2.js b/test/mjsunit/regress/regress-5199.js
similarity index 80%
copy from test/message/regress/regress-4829-2.js
copy to test/mjsunit/regress/regress-5199.js
index eadb653a4b0f7a95dfc5faed6bf7bbd3370b8b8c..818e71a06d30f82ba6e5d1b96b2b25e98d75866c 100644
--- a/test/message/regress/regress-4829-2.js
+++ b/test/mjsunit/regress/regress-5199.js
@@ -2,6 +2,4 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-function tag() {}
-
-`${tag`\xyy`}`;
+assertTrue(/(a[\u1000A])+/i.test('aa'));