[ScheduleDAG] Fix removing edges with weak deps

In SUnit::removePred edges are removed from the Preds and Succs lists before
updating the bookkeeping. This could result in incorrect values for
NumPreds/SuccsLeft and cause WeakPreds/SuccsLeft to underflow, since the
incorrect SDep will be used to update these values.

Reviewed By: rampitec

Differential Revision: https://reviews.llvm.org/D142325