Allow branches without label

[pelletier]

The control flow graph pass does not allow jumps that don't point to a label. In my case, I'd like to jump out of the current function to a different fallback function. I didn't see any reason why this would not be permitted in the current codebase, except if the goal is to force a control flow graph contained within the function explicitly.

This branch is based on https://github.com/mmcloughlin/avo/pull/406, but I'm happy to break it out if it's easier to review.

[pelletier]

@mmcloughlin As I'm going to be reworking #406, do you have any opinion on this change? https://github.com/mmcloughlin/avo/pull/407/commits/e86a5cccf71f7e045286d6b8ca0d8f5f482c211a