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

[Demetrius Kanios via llvm-commits]

QuantumSegfault wrote:

I assumed that using "int" as the key rather than a pointer would help save some minuscule amount of memory (on 64-bit compiler hosts). But it is more readable this way. Either works

But I don't think the alleged ordering issue is a concern. The first thing `WebAssemblyFixIrreducibleControlFlow::makeSingleEntryLoop` does in debug/assertions builds is make sure the blocks after sorted by number are all valid (not -1) and unique. 

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