[BOLT][BAT] Fix encoded NumBasicBlocks

Emit the recorded number of blocks, not the number of basic block
hashes. There might be differences in corner cases (openssl
BN_BLINDING_convert_ex function).

Test Plan:
Updated openssl.test in https://github.com/rafaelauler/bolt-tests/pull/31

Reviewers: rafaelauler, ayermolo, maksfb, dcci

Reviewed By: ayermolo

Pull Request: https://github.com/llvm/llvm-project/pull/87830