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

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

```
[732/1490/3106/5327] Building CXX object lib\Targ...LVMMipsCodeGen.dir\MipsModuleISelDAGToDAG.cpp.obj
C:\Program Files\Microsoft Visual Studio\2022\Professional\VC\Tools\MSVC\14.38.33130\include\optional(883): warning C4018: '>': signed/unsigned mismatch
C:\Program Files\Microsoft Visual Studio\2022\Professional\VC\Tools\MSVC\14.38.33130\include\optional(883): note: the template instantiation context (the oldest one first) is
E:\llvm\llvm-project\llvm\include\llvm/MC/MCInstrItineraries.h(223): note: see reference to function template instantiation 'bool std::operator ><unsigned int,int,0>(const std::optional<unsigned int> &,const _Ty2 &) noexcept(<expr>)' being compiled
        with
        [
            _Ty2=int
        ]
```

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