[llvm] [VPlan] Drop poison-generating flags on induction trunc (PR #168922)

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

> Looking at the reproduce, it looks like the issue is that the induciton phi has been truncated to i8, after which nsw no longer holds. If that's the case, flags should be dropped when optimizing the truncate and forming a narrow IV.

Sorry, I completely messed up the root-cause-analysis! Should be fixed now.

https://github.com/llvm/llvm-project/pull/168922