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

[Jay Foad via llvm-commits]

jayfoad wrote:

> 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.

I should have mentioned that I cannot commit this patch as-is because it causes lit test failures. I think these are all pre-existing problems with LiveDebugVariables that would need to be fixed. There was some prior discussion about this in the previous iteration of this review, #68703. I'd appreciate any help with understanding or fixing the failures!
```
Failed Tests (6):
  LLVM :: CodeGen/Thumb2/pr52817.ll
  LLVM :: DebugInfo/ARM/PR26163.ll
  LLVM :: DebugInfo/COFF/fpo-csrs.ll
  LLVM :: DebugInfo/X86/pr34545.ll
  LLVM :: DebugInfo/X86/spill-indirect-nrvo.ll
  LLVM :: tools/llvm-dwarfdump/X86/LTO_CCU_zero_loc_cov.ll
```

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