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

[Nikita Popov via llvm-commits]

nikic wrote:

> > 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.)
> 
> before modifying the SCEVExpander, are these places legal to modify [here](https://github.com/llvm-mirror/llvm/blob/master/lib/Analysis/ScalarEvolution.cpp#L4434), should we add a class like [this](https://github.com/llvm-mirror/llvm/blob/master/lib/Analysis/ScalarEvolution.cpp#L4470), should we modift [this](https://llvm.org/doxygen/ScalarEvolution_8cpp_source.html#l02382) to

You should not modify any of those places (at least in this PR), only https://github.com/llvm/llvm-project/blob/12b802ac0bc6ddf0742aa3fe8caecd8204d70ca5/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp#L46-L75

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