[llvm] Prevent a crash when a global variable has debug metadata (PR #145918)

[Jeremy Morse via llvm-commits]

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

Urgh. Discussed slightly offline, the fix is correct but it indicates that we've never been able to lookup debug-users of global variables in LLVM, ever, because of the VAM/LocalAsMetadata differences you're fixing here. Which is horrible, but I imagine it's not a common access pattern -- usually we look up the debug-users of a local instruction that's being fiddled with.

Would you be able to repeat the fix/unit-test for findDVRValues please, as it looks like that's vulnerable to the same issue.

https://github.com/llvm/llvm-project/pull/145918