[GlobalISel] Fix CombinerHelper::isPredecessor for same def/use MI.

The doc comment for isPredecessor says:
  Returns true if \p DefMI precedes \p UseMI or they are the same
  instruction.
And dominates relies on that behavior for its own:
  Returns true if \p DefMI dominates \p UseMI. By definition an
  instruction dominates itself.

Make both statements correct by fixing isPredecessor.
Found by inspection.