[llvm] [LiveDebugVariables] Add basic verification (PR #79846)

[Stephen Tozer via llvm-commits]

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

Adding the verification SGTM - LiveDebugVariables can be a moderately hot pass (though LiveDebugValues is the real killer) and PointerIntPair can cause a bit of a slowdown compared to just pointer operations, but I expect this won't have a noticeable impact.

https://github.com/llvm/llvm-project/pull/79846