[llvm] [VPlan] Insert VPBlendRecipes in post order. NFC (PR #201782)

[Luke Lau via llvm-commits]

lukel97 wrote:

> I think at least on the VPlan-level, there may be some changes with the patch.

For the cases where all the incoming values are equal, yeah there should be no end-to-end change because `simplifyBlends` will also fold those blends away to the underlying value.

https://github.com/llvm/llvm-project/pull/201783 will restore the VPlan-level changes at it will be able to see when all incoming values are equal through nested phis. 

https://github.com/llvm/llvm-project/pull/201782