[llvm] [RemoveDIs][DebugInfo][NFC] Add Instruction and convenience functions to DPValue (PR #77896)

[Jeremy Morse via llvm-commits]

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

LGTM, with some inline suggestions. I think this is overall good, however seeing getNextNode etc associated with DPValues makes me twitch, it's a pavlovian response as getNextNode is almost always associated with a gnochange issue in LLVM-IR. I don't think we'll keep this interface forever though, so this should be fine.

https://github.com/llvm/llvm-project/pull/77896