[llvm] [VPlan] Handle live-in extend operands in partial reduction ::computeCost (PR #163175)

[Sander de Smalen via llvm-commits]

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sdesmalen-arm wrote:

I left a comment yesterday on @SamTebbs33' patch that took these changes: https://github.com/llvm/llvm-project/pull/147302#discussion_r2445027350

https://github.com/llvm/llvm-project/pull/163175