[llvm] [VPlan] Simplify live-ins early using SCEV. (PR #155304)

[Ramkumar Ramachandra via llvm-commits]

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

LGTM, thanks! For a follow-up, I will investigate adding to the TypeSwitch in getSCEVExprForVPValue once this lands.

https://github.com/llvm/llvm-project/pull/155304