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

[Luke Lau via llvm-commits]

lukel97 wrote:

> @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

Oh, and then let simplifyRecipes remove the all ones mask from the logical and? That seems like a good idea to avoid creating new masks

Why do we need to do it after all EVL recipes are converted though. Can we do it as we're converting?

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