[BOLT][NFC] Avoid computing BF hash twice in YAML reader (#75096)

We compute BF hashes in `YAMLProfileReader::readProfile` when first
matching profile functions with binary functions, and second time in
`YAMLProfileReader::parseFunctionProfile` during the profile assignment
(we need to do that to account for LTO private functions with
mismatching suffix).

Avoid recomputing the hash if it's been set.