[llvm] [VPlan] Manage FindLastIV start value in ComputeFindLastIVResult (NFC) (PR #132690)

[Luke Lau via llvm-commits]

https://github.com/lukel97 approved this pull request.

LGTM. I think AnyOf reductions will also need the same fix eventually, so maybe this could become something like `ComputeSelectReductionResult`. 

But I don't want to block this chain of PRs, I'm happy if we just start by fixing FindLastIV reductions.

https://github.com/llvm/llvm-project/pull/132690