[mlir] Use dyn_cast instead of cast

Author: CoTinker

mlir/test/lib/Conversion/MathToVCIX/TestMathToVCIXConversion.cpp

@@ -25,7 +25,7 @@ namespace {
 /// according to LLVM's encoding:
 /// https://lists.llvm.org/pipermail/llvm-dev/2020-October/145850.html
 static std::pair<unsigned, VectorType> legalizeVectorType(const Type &type) {
-  VectorType vt = cast<VectorType>(type);
+  VectorType vt = dyn_cast<VectorType>(type);
   // To simplify test pass, avoid multi-dimensional vectors.
   if (!vt || vt.getRank() != 1)
     return {0, nullptr};