evp/e_aes.c: replace calls to one-liners with references in GCM.

Even though calls can be viewed as styling improvement, they do come
with cost. It's not big cost and shows only on short inputs, but it is
measurable, 2-3% on some platforms.

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6312)