[llvm] [LV][EVL] Reimplement method for extracting new mask. nfc (PR #156827)

[Florian Hahn via llvm-commits]

https://github.com/fhahn commented:

> @lukel97 @alexey-bataev @fhahn Just after I submitted the PR, I suddenly thought of another approach. What if we simply replace the header mask with an all-true mask after all EVL recipes are converted? Wouldn’t that be simpler? :D

Yes that would be better, if it is doable.

But I guess we need to make sure all users of the mask got converted to EVL-based recipes?

https://github.com/llvm/llvm-project/pull/156827