[llvm] [VPlan] Replace PhiR operand of ComputeRdxResult with VPIRFlags. (PR #174026)

[Mel Chen via llvm-commits]

https://github.com/Mel-Chen approved this pull request.

LGTM.
BTW, this patch should be marked as NFC if my understanding is correct.

https://github.com/llvm/llvm-project/pull/174026