[llvm] [IR] Fix crashes caused by #85592 (PR #87169)

[Shilei Tian via llvm-commits]

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@@ -28,8 +28,9 @@ bool Operator::hasPoisonGeneratingFlags() const {
     return OBO->hasNoUnsignedWrap() || OBO->hasNoSignedWrap();
   }
   case Instruction::Trunc: {
-    auto *TI = dyn_cast<TruncInst>(this);
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shiltian wrote:

This style is really bad. Either using a `cast` directly w/o a check or check the result before use.

https://github.com/llvm/llvm-project/pull/87169