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

[Sander de Smalen via llvm-commits]

================

----------------
sdesmalen-arm wrote:

(Note that this comment relates only to #147302 because that case couldn't be tested without the changes added in that PR, so there's nothing to do for that case in this PR)

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