[llvm] Add DebugSSAUpdater class to track debug value liveness (PR #134055)

[Stephen Tozer via llvm-commits]

SLTozer wrote:

Just noting that the undef deprecator comment above is incorrect - this code doesn't contain instances of `UndefValue` but uses the text "undef" as in the term typically used in debug info to refer to undefined values - whether or not this needs to change, I'll leave to reviewers.

https://github.com/llvm/llvm-project/pull/134055