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

[Luke Lau via llvm-commits]

lukel97 wrote:

Ping for more discussion. I've put up a PR to show how the loop vectorizer would use it in https://github.com/llvm/llvm-project/pull/212717. It also shows how we get more accurate costing by sinking the `llvm.experimental.cttz.elts` to the middle block, since in an early exit loop it has multiple users, despite one of the users folding it into `brkb` on SVE.

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