TargetInstrInfo, TargetSchedule: fix non-NFC parts of 9468de48 (#74338)

Follow up on a post-commit review of 9468de48 (TargetInstrInfo: make
getOperandLatency return optional (NFC)) by Bjorn Pettersson to fix a
couple of things that are not NFC:

- std::optional<T>::operator<= returns true if the first operand is a
std::nullopt and second operand is T. Fix a couple of places where we
assumed it would return false.
- In TargetSchedule, computeInstrCost could take another codepath,
returning InstrLatency instead of DefaultDefLatency. Fix one instance
not accounting for this behavior.