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

[Ramkumar Ramachandra via llvm-commits]

artagnon wrote:

I'm somewhat confused by the build log. The only places the error could possibly be coming from are:

```
  if (UseCycle > *DefCycle + 1)
    return std::nullopt;
```

Here, `*DefCycle` is `unsigned`, and `unsigned + 1 = unsigned`, so this shouldn't qualify.

The other place is:

```
    UseCycle = *DefCycle - *UseCycle + 1;
    if (UseCycle > 0 &&
        hasPipelineForwarding(DefClass, DefIdx, UseClass, UseIdx))
```

`UseCycle` is unsigned, and although MSVC could have inferred that its assignment would produce a negative value, it would technically be an unsigned-wrap. Besides, the error is coming from `>`, and I don't see how `UseCycle > 0` could possibly qualify.

Could you try fixing up the first instance with an unsigned-cast, try it on MSVC, and tell me if the error still persists? If it doesn't, I'll push a fix.

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