Refactor basic block annotation into a separate function

[boomanaiden154]

This patch refactors the annotation process for an individual basic block into a separate function. This is intended to make future tasks easier (like introducing threading to the script).

[boomanaiden154]

This is stacked on top of https://github.com/google/gematria/pull/66 to avoid merge conflicts when things start landing and to make it easier for me to keep all the fixes in a branch for testing purposes.

[ondrasej]

Eventually, it would be best to move the functionality to a library, and add unit tests for it.