[llvm] [LiveDebugVariables] Add basic verification (PR #68703)

[Jay Foad via llvm-commits]

jayfoad wrote:

> It would be better if I could directly query SlotIndexes to ask "is this slot still live?" (under #ifndef NDEBUG). I'll work on implementing something like that.

I have reworked the patch along those lines. It currently causes some lit tests failures in my build. I would suggest that these are all pre-existing problems that really ought to be fixed.
```
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/68703