TargetInstrInfo: make getOperandLatency return optional (NFC) (#73769)

getOperandLatency has the following behavior: it returns -1 as a special
value, negative numbers other than -1 on some target-specific overrides,
or a valid non-negative latency. This behavior can be surprising, as
some callers do arithmetic on these negative values. Change the
interface of getOperandLatency to return a std::optional<unsigned> to
prevent surprises in callers. While at it, change the interface of
getInstrLatency to return unsigned instead of int.

This change was inspired by a refactoring in
TargetSchedModel::computeOperandLatency.