Use IsA() macro in define.c, for sake of consistency.

Use IsA() macro in define.c, for sake of consistency.

Commit 63d1b1cf7f1 replaced a direct nodeTag() comparison with the IsA()
macro in copy.c, but a similar direct comparison remained in define.c.

This commit replaces that comparison with IsA() for consistency.

Author: Fujii Masao <masao.fujii@gmail.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Shinya Kato <shinya11.kato@gmail.com>
Reviewed-by: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Discussion: https://postgr.es/m/CAHGQGwGjWGS89_sTx=sbPm0FQemyQQrfTKm=taUhAJFV5k-9cw@mail.gmail.com