[llvm] [VPlan] Fold common edges away in convertPhisToBlends (PR #150368)

[Luke Lau via llvm-commits]

lukel97 wrote:

> It looks correct to me. BTW, I don't have a particular preference, but I do have one question: as an optimization, should this be implemented in VPlanTransform::optimize?

It's possible to do it on unnormalized blends in `simplifyBlends`, but I was exploring removing blend recipes entirely in #150369, and doing it here means we can directly emit select VPInstructions later.

It's generally not possible to do it once it's normalized or converted to selects because we lose the first mask, and with it the information that the first mask also shares the common edge.

https://github.com/llvm/llvm-project/pull/150368