[llvm] [IR][RFC] Add @llvm.mask.beforefirst intrinsic (PR #203874)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

Thanks for the update, given the discussion I think adding the intrinsic is fine.

I did some fuzzing and it looks like there are a few crashes when lowering on X86, not sure if those are easy to fix.

https://github.com/llvm/llvm-project/pull/203874