[ScheduleDAG] Fix unfolding of SUnits to already existent nodes.

Summary:
If unfolding an SUnit results in both load or the operation using it which
already exist in the DAG, abort the unfold if they are already scheduled.
If not, make sure we don't add duplicate dependencies.

This fixes PR37916.

Reviewers: davide, eli.friedman, fhahn, bogner

Subscribers: MatzeB, hiraditya, llvm-commits

Differential Revision: https://reviews.llvm.org/D48666

llvm-svn: 337409