[llvm] [WebAssembly] Replace Reachability with SCCs in Irreducible CFG Fixer (PR #179722)

[Demetrius Kanios via llvm-commits]

QuantumSegfault wrote:

Looks like this has already been done in a different form elsewhere in LLVM!

https://github.com/llvm/llvm-project/blob/main/llvm/include/llvm/ADT/SCCIterator.h

```cpp
/// This builds on the llvm/ADT/GraphTraits.h file to find the strongly
/// connected components (SCCs) of a graph in O(N+E) time using Tarjan's DFS
/// algorithm.
```

--------

I was looking for information on how to generate irreducible CFG, when I found a blog post about Cheerp, with a passing mention of _fixing_ irreducible control flow. https://labs.leaningtech.com/blog/control-flow#fix-the-irreducible-control-flow

So I took a look:
https://github.com/leaningtech/cheerp-compiler/blob/master/llvm/lib/CheerpUtils/FixIrreducibleControlFlow.cpp

--------

So do we want to try to use the existing SCCIterator instead of rolling our own implementation of the algorithm? Not sure if it'll be any better. Might end up being less optimized given our constraints/assumptions...but maybe we can have more confidence in it as it's been tested more thoroughly?

https://github.com/llvm/llvm-project/pull/179722