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 <martin@meltin.net>
Reviewed-by: Amitay Isaacs <amitay@gmail.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15798)