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

`DPLabel`, the RemoveDI version of `llvm.dbg.label`, was landed recently
at the same time the RemoveDIs IR printing and verifier patches were
landing. The patches were updated to not miscompile, but did not have
full-featured and correct support for DPLabel built in; this patch makes
the remaining set of changes to enable DPLabel support.