[mlir][tosa] Fix a crash in PadOp::fold (#114921)

This PR fixes a crash when padding of `tosa.pad` is not dense elements.
Fixes #114762.

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();
     }
   }

mlir/test/Dialect/Tosa/canonicalize.mlir

@@ -217,6 +217,17 @@ func.func @pad_noop(%arg0: tensor<?x?xf32>) -> tensor<?x?xf32> {
 
 // -----
 
+// CHECK-LABEL: @pad_noop_padding_mismatch_nofold
+func.func @pad_noop_padding_mismatch_nofold(%arg0: tensor<?x?xf32>) -> tensor<?x?xf32> {
+  // CHECK: %[[PAD:.+]] = tosa.pad
+  // CHECK: return %[[PAD]]
+  %0 = "tosa.const"() { value = dense_resource<__elided__> : tensor<2x2xi32>} : () -> tensor<2x2xi32>
+  %1 = tosa.pad %arg0, %0 : (tensor<?x?xf32>, tensor<2x2xi32>) -> tensor<?x?xf32>
+  return %1 : tensor<?x?xf32>
+}
+
+// -----
+
 // CHECK-LABEL: @pad_noop_type_mismatch_nofold
 func.func @pad_noop_type_mismatch_nofold(%arg0: tensor<10xf32>) -> tensor<?xf32> {
   // CHECK: %[[PAD:.+]] = tosa.pad