bn: Drop unnecessary use of r9
bn: Drop unnecessary use of r9

This is done in other versions due to the possibility of an early
return.  However, there is no early return here.

Signed-off-by: Martin Schwenke <54669547a225ff20cba8b75a4adca540eef25858@meltin.net>
Reviewed-by: Amitay Isaacs <a505e7c038746dbeb3edcd8d146dbd515432801f@gmail.com>
Reviewed-by: Tomas Mraz <2bc6038c3dfca09b2da23c8b6da8ba884dc2dcc2@openssl.org>
Reviewed-by: Paul Dale <ddec78389666b4f7c9b4d110380c848489a3e4aa@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15798)

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl