[llvm] [VPlan] Extract IV exit value from vector loop if predicates are needed. (PR #200020)

[Mel Chen via llvm-commits]

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

LGTM. 
BTW, could this eventually be resolved by using `VPDerivedIV(start, ExitCount, step)`, where `ExitCount = sub(ResumeTC, 1)`?

https://github.com/llvm/llvm-project/pull/200020