[ScheduleDAG] Deal with already scheduled loads in ScheduleDAG.

Summary:
If we attempt to unfold an SUnit in ScheduleDAG that results in
finding an already scheduled load, we must should abort the
unfold as it will not improve scheduling.

This fixes PR32610.

Reviewers: jmolloy, sunfish, bogner, spatel

Subscribers: llvm-commits, MatzeB

Differential Revision: https://reviews.llvm.org/D32911

llvm-svn: 304321