[mlir][arith] Only fold splats for static shape result types (#93102)

This prevents an assertion when constructing the DenseElementsAttr
result, where the passed-in type is expected to have a static shape.

Fixes #92057

Author: KB9

mlir/include/mlir/Dialect/CommonFolders.h

@@ -298,7 +298,10 @@ Attribute constFoldCastOp(ArrayRef<Attribute> operands, Type resType,
         calculate(op.getSplatValue<ElementValueT>(), castStatus);
     if (!castStatus)
       return {};
-    return DenseElementsAttr::get(cast<ShapedType>(resType), elementResult);
+    auto shapedResType = cast<ShapedType>(resType);
+    if (!shapedResType.hasStaticShape())
+      return {};
+    return DenseElementsAttr::get(shapedResType, elementResult);
   }
   if (auto op = dyn_cast<ElementsAttr>(operands[0])) {
     // Operand is ElementsAttr-derived; perform an element-wise fold by

mlir/test/Dialect/Arith/canonicalize.mlir

@@ -2950,6 +2950,14 @@ func.func @unsignedExtendConstantResource() -> tensor<i16> {
   return %ext : tensor<i16>
 }
 
+// Just checks that this doesn't crash.
+// CHECK-LABEL: @signedExtendSplatAsDynamicShape
+func.func @signedExtendSplatAsDynamicShape() -> tensor<?xi64> {
+  %splat = arith.constant dense<5> : tensor<2xi16>
+  %extsplat = arith.extsi %splat : tensor<2xi16> to tensor<?xi64>
+  return %extsplat : tensor<?xi64>
+}
+
 // CHECK-LABEL: @extsi_i0
 //       CHECK:   %[[ZERO:.*]] = arith.constant 0 : i16
 //       CHECK:   return %[[ZERO]] : i16