[llvm] [RemoveDIs][DebugInfo] Verifier and printing fixes for DPLabel (PR #83242)

[Stephen Tozer via llvm-commits]

SLTozer wrote:

> LGTM, for full coverage you might want to add test coverage for the verifier errors too. I can't remember whether we've already done that for other RemoveDI records.

We've not; with that said, right now all the verifier tests are performed by `llvm-as`, and it would be good to keep that in place - in particular because it would make all the testing automatic, i.e. no large addition of new files that are almost identical to the old files, just an immediate coverage switchover. Doing so requires `llvm-as` to support conversion to the new format however, with or without a flag - would that be suitable as a separate patch, or would it be better to do the test cloning for now (replacing `llvm-as` with `opt -verify`)?

https://github.com/llvm/llvm-project/pull/83242