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

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goldsteinn wrote:

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

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