I believe the actual opcode does not matter because the AVR architecture is a Harvard architecture that does not support writing to program memory. Therefore, debuggers and emulators provide hardware breakpoints. But for some reason, this opcode must be defined or else LLDB will crash with an assertion error.
I would like to add a test case for this, but I'm not quite sure how to do that. It appears that to trigger the crash (fixed by this patch), there needs to be a gdb-remote.