s390x/tcg: Simplify vflr64() handling

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: David Hildenbrand <david@redhat.com>
Message-Id: <20210608092337.12221-10-david@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>