[llvm] [LV][EVL] Simplify EVL recipe transformation by using a single EVL mask. nfc (PR #152479)

[Luke Lau via llvm-commits]

https://github.com/lukel97 approved this pull request.

LGTM. This should also make it easier to split up the parts needed for correctness and which parts are just about optimizing away the header mask. 

As an aside, its weird to begin with that there might be multiple header masks? From a quick check it only appears to happen when the data tail folding style is used, and we generate both a VPInstruction::ActiveLaneMask and a `icmp ule IV, BTC`?. I'll see if we can change this so that we only have one header mask max.

https://github.com/llvm/llvm-project/pull/152479