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

[Luke Lau via llvm-commits]

lukel97 wrote:

> LGTM, thanks!
> 
> It's a bit of a shame we have to do this there, but given that blends don't store the mask of their first operand, I don't see any way around this

Unnormalized masks store the mask of their first operand though. For completeness sake I tried out what @Mel-Chen had suggested and did the optimisation on unnormalized VPBlendRecipes in simplifyBlends. 

My thinking was that doing this in simplifyBlends means that simplifyRecipes will have had a chance to kick in, and maybe we would catch more cases, but it didn't actually make a difference to any of the tests in the end, even when stacked on top with the additional simplifications in https://github.com/llvm/llvm-project/pull/155383. 

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