[llvm] [ci] Don't add more check targets if "check-all" is present (PR #110277)

[Louis Dionne via llvm-commits]

ldionne wrote:

> Can't we do something along the lines of `check-compiler-rt` instead of `check-all`? That would be much simpler than your change.

Can you expand on this suggestion? I don't quite understand. What does `check-compiler-rt` do?

https://github.com/llvm/llvm-project/pull/110277