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

[Francesco Petrogalli via llvm-commits]

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@@ -202,24 +204,25 @@ class InstrItineraryData {
   /// Compute and return the use operand latency of a given itinerary
   /// class and operand index if the value is produced by an instruction of the
   /// specified itinerary class and def operand index.
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fpetrogalli wrote:

Here too, explain why we want to use optional

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