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

[Simon Pilgrim via llvm-commits]

RKSimon wrote:

@artagnon I'm seeing a lot of signed/unsigned warnings on MSVC builds (and we build with WX/Werror by default) - please can you take a look?

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