[llvm] [IR] Add nowrap flags for trunc instruction (PR #85592)

[Nikita Popov via llvm-commits]

nikic wrote:

One more place you need to update for correctness reasons is InstCombineSimplifyDemanded: You need to drop poison generating flags when doing demanded bits simplification of trunc now. (This should be tested.)

Another place to update is SCEVExpander, which backs up poison-generating flags. Needs an adjustment to handle these on trunc now. (Don't think this needs a test, it's pretty tricky to do that.)

https://github.com/llvm/llvm-project/pull/85592