[llvm] TargetInstrInfo: make getOperandLatency return optional (NFC) (PR #73769)

[Francesco Petrogalli via llvm-commits]

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@@ -162,18 +163,19 @@ class InstrItineraryData {
     return Latency;
   }
 
-  /// Return the cycle for the given class and operand.  Return -1 if no
-  /// cycle is specified for the operand.
-  int getOperandCycle(unsigned ItinClassIndx, unsigned OperandIdx) const {
+  /// Return the cycle for the given class and operand. Return std::nullopt if
+  /// no cycle is specified for the operand.
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fpetrogalli wrote:

How about
```suggestion
  /// the information is not specified for the operand.
```

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