[llvm] [InstCombine] Fold `sext(trunc nsw)` and `zext(trunc nuw)` (PR #88609)

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YanWQ-monad wrote:

> your proofs seem overly complex. I think the following suffice: https://alive2.llvm.org/ce/z/WbDanN

That's because Alive2 didn't support `nsw` and `nuw` at that time, then I used some tricks to emulate it. :) And yes, I should update the proofs. Thanks for the reminder.

https://github.com/llvm/llvm-project/pull/88609