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

`shouldUseDebugInstrRef` can return different value than
`useDebugInstrRef`, since the first depends on opt level which can
change. Inconsistent usage can lead to errors later.

I believe that using `should...` instead of `use...` here is a result of
a minor error during this:
https://github.com/llvm/llvm-project/pull/94149/files#diff-8ec547e1244562c5837ed180dd9bed61b3cd960ef90bb6002ea2db41a67ed693

Notice how before the change `InstrRef` is assigned value from
`should...` *before* the opt change. Now, it's done after -- opt change
happens here:
```c
bool SelectionDAGISelLegacy::runOnMachineFunction(MachineFunction &MF) {
...
  // Decide what flavour of variable location debug-info will be used, before
  // we change the optimisation level.
  MF.setUseDebugInstrRef(MF.shouldUseDebugInstrRef());
....

  return Selector->runOnMachineFunction(MF);
}
```

Then `runOnMachineFunction` uses `should...`, which after opt change may
return different value than it did previously.