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

[Andrei Elovikov via llvm-commits]

eas wrote:

I guess this is fine for now, so I definitely wouldn't want to block it, but I have a feeling RPOT would be better if we ever try to preserve some "uniform" branches as that would require placing SSA-phis sometimes.

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