[llvm] [VPlan] Expand AddRecs in VPSCEVExpander (PR #209921)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

Figured out several things and updated the commit message: we expand AddRecs to VPInstructions exactly when it is safe, and fall back to the IR expander when the AddRec refers to a loop no longer in the Plan.

https://github.com/llvm/llvm-project/pull/209921