Implement Relooper algorithm

[aborgna-q]

Transform unstructured control flow into Conditionals and TailLoops (as much as possible).

We may want to start by implementing some simpler algorithms for straight-forward cases, to avoid the unnecessary structure of relooper's output.

See

• Emscripten paper: https://github.com/emscripten-core/emscripten/blob/main/docs/paper.pdf
• azakai's blogpost: https://mozakai.blogspot.com/2012/05/reloop-all-blocks.html

We may actually implement the Stackifier algorithm instead.

• https://github.com/llvm/llvm-project/blob/main/llvm/lib/Target/WebAssembly/WebAssemblyCFGStackify.cpp
• See this post for discussion on relooper drawbacks vs stackify: https://medium.com/leaningtech/solving-the-structured-control-flow-problem-once-and-for-all-5123117b1ee2

Note that we have mostly all the structured control flow primitives required, except for multi-level breaks (we'll probably have to simulate that by returning sums).

This should probably be implemented in hugr-passes instead of here.