[llvm] [VPlan] Delay header mask creation until after blend recipes generation (PR #178623)

[Luke Lau via llvm-commits]

lukel97 wrote:

> Ideally, we should have a way to avoid blending in the header mask when it isn’t actually needed. At least for complex masks, we can go with the post-dominator–based approach https://github.com/llvm/llvm-project/pull/173265 for now. :)

https://github.com/llvm/llvm-project/pull/173265 seems like a more general approach, and sounds like a good direction to me

https://github.com/llvm/llvm-project/pull/178623