[llvm] [LiveDebugVariables] Add tests for stale SlotIndexes (PR #225375)

[Jeremy Morse via llvm-commits]

jmorse wrote:

Soft LGTM, I'm glad to see any increase in coverage of what these passes do, thank you! My only suggestion would be that this'd be a bit clearer if written as a MIR test (should be easily convertable with `llc -stop-before=livedebugvars --simplify-mir`), it'd reduce un-necessary test coverage and make it easier to pin down exactly what happens.

https://github.com/llvm/llvm-project/pull/225375