[llvm] Only guard loop metadata that has non-debug info in it (PR #118825)

[Dominik Steenken via llvm-commits]

dominik-steenken wrote:

I have added an additional check that makes sure that when we optimize away a loop latch with only debug info, that that debug info then isn't used to overwrite otherwise useful metadata in the predecessor.
I also expanded upon the existing tests.

https://github.com/llvm/llvm-project/pull/118825