[SECP256K1] Optimization: special-case zero modulus limbs in modinv64
Summary:
Both the field and scalar modulus can be written in signed{30,62}
notation with one or more zero limbs. Make use of this in the update_de
function to avoid a few wide multiplications when that is the case.
This doesn't appear to be a win in the 32-bit implementation, so only
do it for the 64-bit one.Partial backport of secp256k1#831:
https://github.com/bitcoin-core/secp256k1/pull/831/commits/9164a1b6582e2fc833c760a3403d26b9b0b3b7b3
Depends on D9408.
Test Plan:
ninja check-secp256k1
Reviewers: #bitcoin_abc, majcosta
Reviewed By: #bitcoin_abc, majcosta
Differential Revision: https://reviews.bitcoinabc.org/D9409