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

[Luke Lau via llvm-commits]

lukel97 wrote:

> 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.

I've opened up https://github.com/llvm/llvm-project/pull/152489 for this

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