[VPlan] Simplify PredPHI LiveIn -> LiveIn (#142271)
5f39be5 ([VPlan] Use InstSimplifyFolder instead of TargetFolder) updated
simplifyRecipe to fold live-ins to Values that are not necessarily
Constant, but forgot to update the corresponding PredPHI folder, which
still folds PredPHI constant -> constant. Update it to fold PredPHI
LiveIn -> LiveIn.
Fixes #141968.
