Hi security_enthusiast,
The best way will be checking the release notes of each version in case you need to check all those version, now if you are only trying to find the bugs since you would like to do an upgrade from 8.4.2 to 9.1.7.7** you can just check the release notes of 9.1.7 since the bugs affecting 8.4.2 are not the same that the ones on 9.1.7 (you could have a bug that is affecting both versions as well).
Search for open bugs:
http://www.cisco.com/c/en/us/td/docs/security/asa/asa91/release/notes/asarn91.html
**Cisco.com suggested latest interim version
Hope this info helps!!
Rate if helps you!!
-JP-