[function-attrs] Refactor code to handle shorter code with early exits.

[function-attrs] Refactor code to handle shorter code with early exits.

No functionality changed here, but the indentation is substantially
reduced and IMO the code is much easier to read. I've also added some
helpful comments.

This is just a clean-up I wrote while studying the code, and that has
been in my backlog for a while.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@251381 91177308-0d34-0410-b5e6-96231b3b80d8