[llvm] VectorCombine: fix logical error after m_Trunc match (PR #91201)

[Simon Pilgrim via llvm-commits]

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@@ -1961,15 +1961,14 @@ bool VectorCombine::foldTruncFromReductions(Instruction &I) {
   if (!match(ReductionSrc, m_OneUse(m_Trunc(m_Value(TruncSrc)))))
     return false;
 
-  auto *Trunc = cast<CastInst>(ReductionSrc);
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RKSimon wrote:

dyn_cast instead?

https://github.com/llvm/llvm-project/pull/91201