Fix update_slice visibility issue (#915)

# Pull Request

## What problem does this PR solve?
If operand is public, update is secret. We should not cast update to
public.

Issue Number: Fixed #

## Possible side effects?

- Performance: N/A

- Backward compatibility: N/A

libspu/kernel/hal/shape_ops.cc

@@ -43,10 +43,10 @@ Value update_slice(SPUContext* ctx, const Value& in, const Value& update,
   SPU_TRACE_HAL_DISP(ctx, in, start_indices);
 
   if (in.storage_type() != update.storage_type()) {
-    auto u =
-        _cast_type(ctx, update, in.storage_type()).setDtype(update.dtype());
-
-    return update_slice(ctx, in, u, start_indices);
+    auto ct = _common_type(ctx, update.storage_type(), in.storage_type());
+    auto u = _cast_type(ctx, update, ct).setDtype(update.dtype());
+    auto i = _cast_type(ctx, in, ct).setDtype(in.dtype());
+    return update_slice(ctx, i, u, start_indices);
   }
 
   return _update_slice(ctx, in, update, start_indices).setDtype(in.dtype());