[llvm] [WebAssembly] Implement the wide-arithmetic proposal (PR #111598)

[Alex Crichton via llvm-commits]

alexcrichton wrote:

cc @dschuff @tlively 

I'll also call out that this is my first substantial (ish) contribution to LLVM, and I'm very green when it comes to backend lowerings, TableGen, etc. Please point out any non-idiomatic things or similar! I'm not certain I reached the correct conclusion that C++ is required for these lowerings instead of being able to use TableGen, so an extra pair of eyes on that would be much appreciated.

https://github.com/llvm/llvm-project/pull/111598