[NFC][mlir][vector] Handle potential static cast assertion. (llvm#152957)

In FoldArithToVectorOuterProduct pattern, static cast to vector type
causes assertion when a scalar type was encountered. It seems the author
meant to have a dyn_cast instead.

This NFC patch handles it by using dyn_cast.

Author: shahidact

mlir/lib/Dialect/Vector/Transforms/VectorTransforms.cpp

@@ -2274,7 +2274,7 @@ struct FoldArithToVectorOuterProduct : public OpRewritePattern<MulOpType> {
 
   LogicalResult matchAndRewrite(MulOpType mulOp,
                                 PatternRewriter &rewriter) const override {
-    auto resType = llvm::cast<VectorType>(mulOp.getResult().getType());
+    auto resType = llvm::dyn_cast<VectorType>(mulOp.getResult().getType());
     if (!resType)
       return failure();
     if (resType.getRank() != 2)