[mlir][tosa] Fix a crash when padding of tosa.pad is not dense elements

This PR fixes a crash  when padding of `tosa.pad` is not dense elements.

Author: CoTinker

mlir/lib/Dialect/Tosa/IR/TosaCanonicalizations.cpp

@@ -878,8 +878,9 @@ OpFoldResult ReshapeOp::fold(FoldAdaptor adaptor) {
 OpFoldResult PadOp::fold(FoldAdaptor adaptor) {
   // If the pad is all zeros we can fold this operation away.
   if (adaptor.getPadding() && getInput1().getType() == getType()) {
-    auto densePad = llvm::cast<DenseElementsAttr>(adaptor.getPadding());
-    if (densePad.isSplat() && densePad.getSplatValue<APInt>().isZero()) {
+    auto densePad = llvm::dyn_cast<DenseElementsAttr>(adaptor.getPadding());
+    if (densePad && densePad.isSplat() &&
+        densePad.getSplatValue<APInt>().isZero()) {
       return getInput1();
     }
   }