[llvm] [VPlan] Don't reset canonical IV start value. (PR #161589)

[via llvm-commits]

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

LGTM, thanks for extracting from #156262, copied over related minor comments.

Title might be a bit more accurate, as the start value of the canonical IV continues to be  reset, but later. Perhaps "Delay reset of epilog canonical IV start value". Also, NFC?

https://github.com/llvm/llvm-project/pull/161589