[llvm] [VPlan] Add ComputeFindLastIVResult opcode (NFC). (PR #132689)

[Luke Lau via llvm-commits]

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

LGTM, I think we should also eventually move AnyOf reductions to use this too, but happy to defer that after this chain of PRs.

https://github.com/llvm/llvm-project/pull/132689