[llvm] [VPlan] Manage Sentinel value for FindLastIV in VPlan. (PR #142291)

[David Sherwood via llvm-commits]

https://github.com/david-arm approved this pull request.

LGTM! Seems reasonable and looks like we're closer to removing all dependencies on RdxDesc in the vplan recipes code.

https://github.com/llvm/llvm-project/pull/142291