Unify BN_rshift design
This commit aims at refactoring the `BN_rshift` by making it a wrapper around `bn_rshift_fixed_top`, in order to match the current design of `BN_lshift`, as suggested in the discussion at https://github.com/openssl/openssl/pull/10122#discussion_r332474277 . As described in the code, by refactoring this function, `BN_rshift` provides a constant-time behavior for sufficiently[!] zero-padded inputs under the following assumptions: `|n < BN_BITS2|` or `|n / BN_BITS2|` being non-secret. Notice that `BN_rshift` returns a canonical representation of the BIGNUM, if a `fixed_top` representation is required, the caller should call `bn_rshift_fixed_top` instead. Reviewed-by: NNicola Tuveri <nic.tuv@gmail.com> Reviewed-by: NMatt Caswell <matt@openssl.org> (Merged from https://github.com/openssl/openssl/pull/10196)
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