[llvm] [RemoveDIs][DebugInfo] Handle RAUW from dead constants (PR #80837)

[Jeremy Morse via llvm-commits]

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

LGTM -- there are a few nullptrs leaking in from abnormal freeing-of-values paths, it'll be good to have this catch-all.

https://github.com/llvm/llvm-project/pull/80837