[mlir][Tensor] Make `TilingInterface` implementation only return handle to the created `pad` operation.

[mravishankar]

Summary

Pad tiling implementation only needs to return the tiled pad
operation. The rest of the generated code is related to handling
boundary conditions.

mlir/lib/Dialect/Tensor/IR/TensorTilingInterfaceImpl.cpp

auto result = b.create<scf::IfOp>(
           thenOp = createGenerateOp();
           b.create<scf::YieldOp>(loc, castResult(thenOp->getResult(0)));
         },
         /*elseBuilder=*/
         [&](OpBuilder &b, Location loc) {
           elseOp = createPadOfExtractSlice();
           b.create<scf::YieldOp>(loc, castResult(elseOp->getResult(0)));
         });
-    return TilingResult{{result}, SmallVector<Value>(result->getResults())};
+    return TilingResult{{elseOp}, SmallVector<Value>(result->getResults())};
   }
 
   Operation *newPadOp = createPadOfExtractSlice();
   return TilingResult{{newPadOp}, {castResult(newPadOp->getResult(0))}};
 }
 
 void mlir::tensor::registerTilingInterfaceExternalModels(
     DialectRegistry &registry) {