Refactor attributes checking codes shared between Add and Merge functions

Looks like it's heading in the right direction at a high level. One thing that would be nice (but perhaps not easy to do) would be to see how much attribute checking code can be refactored to share between the Add and Merge functions here. They're doing the same things notionally (as best I saw), so it'd be nice to only write that logic once if possible. However, that might require more refactoring that's better left until later.

_Originally posted by @AaronBallman in https://github.com/intel/llvm/pull/5782#pullrequestreview-907166589_