[llvm] [AMDGPU][test] fix the error case in update_mc_test_check script (PR #112731)

[Ivan Kosarev via llvm-commits]

kosarev wrote:

> > Needs a test using the `--llvm-mc-binary` option?
> 
> I am not sure if we can do this in the test here? since it requires a mc-binary location

When I use `--llvm-mc-binary`, which is my default way of using it, the script changes the `NOTE: Assertions have been autogenerated by ...` line to mention the `--llvm-mc-binary` option. This doesn't happen when I update llc tests.

https://github.com/llvm/llvm-project/pull/112731