I think we need wrappers for asserts in C2 to provide information about failure which may happens only once during testing.
Example: JDK-8262093

The assert code is this:

    } else if (mem->is_ArrayCopy()) {
      done = true;
    } else {
      assert(false, "unexpected node");
    }

Without reproducer (which may impossible to make) it is hard to know which wrong Ideal node was there.

There are a lot of other similar cases in C2 code. I think we should fix/improve assertion code.