[llvm] [SDAG] Use useDebugInstrRef instead of shouldUseDebugInstrRef (PR #160686)

[Mikołaj Piróg via llvm-commits]

mikolaj-pirog wrote:

> I think it is worth mentioning that the main problem happens while using opt-bisect. When the optimization level changes. My concern here is whether we need `shouldUseDebugInstrRef` at all? Or maybe we need to check whether we are bisecting in `shouldUseDebugInstrRef` and return a consistent result for it?
> 
> UPD: We need some testing as well...

My understanding of the `shouldUseDebugInstrRef` is that it's a helper that's used to set `UseInstrRef`; it should never be used outside of initialization (and indeed the usage removed in this PR is the only one)

https://github.com/llvm/llvm-project/pull/160686