[llvm] TargetSchedule: factor out code in computeOperandLatency (NFC) (PR #73769)

[Francesco Petrogalli via llvm-commits]

================
@@ -186,21 +189,16 @@ unsigned TargetSchedModel::computeOperandLatency(
       unsigned DefClass = DefMI->getDesc().getSchedClass();
       OperLatency = InstrItins.getOperandCycle(DefClass, DefOperIdx);
     }
-    if (OperLatency >= 0)
-      return OperLatency;
-
-    // No operand latency was found.
-    unsigned InstrLatency = TII->getInstrLatency(&InstrItins, *DefMI);
 
     // Expected latency is the max of the stage latency and itinerary props.
     // Rather than directly querying InstrItins stage latency, we call a TII
     // hook to allow subtargets to specialize latency. This hook is only
     // applicable to the InstrItins model. InstrSchedModel should model all
     // special cases without TII hooks.
-    InstrLatency =
-        std::max(InstrLatency, TII->defaultDefLatency(SchedModel, *DefMI));
-    return InstrLatency;
+    return OperLatency >= 0 ? OperLatency
----------------
fpetrogalli wrote:

If we change the interface of the TTI, this would become something like
```suggestion
    return OperLatency.has_value() ? OperLatency
```
which would prevent readers of the code asking the question: what if the latency is negative?

https://github.com/llvm/llvm-project/pull/73769