[llvm] [InstrRef] Fix mismatch between LiveDebugValues and salvageCopySSA (PR #124233)

[Jeremy Morse via llvm-commits]

https://github.com/jmorse approved this pull request.

LGTM; I think we can skip writing a test as the failure isn't really an illegal output from the compiler, it's an inconsistency between the way these two passes are implemented. We might test certain things are considered copies, but it'd be exhausting to test that everything considered a copy by salvageCopySSA is considered a copy by LiveDebugValues.

A bigger question is "how do we prevent this happening in the future", and I'm not sure we need to as (IIRC) these are the only two parts of instr-ref that care about copies.

https://github.com/llvm/llvm-project/pull/124233