[SECP256K1] Use bit ops instead of int mult for constant-time logic in gej_add_ge

Summary:

Nothing is wrong with the current implementation. But it looks out of
place because we generally prefer bit operations because they're faster
and are more likely constant-time.

Backport of secp256k1#882

Test Plan:

ninja check-secp256k1

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9388