[MachineCSE][NFC]: Refactor and comment on preventing CSE for isConvergent instrs

- Move the code preventing CSE of `isConvergent` instrs into
  `ProcessBlockCSE` (from `isProfitableToCSE`)
- Add comments explaining why `isConvergent` is used to prevent
  CSE of non-local instrs in MachineCSE and the new test