Use iterators to iterate through the Preds array instead of

Use iterators to iterate through the Preds array instead of
an index. This code is on the hot-path because the current
way SDep edges are uniqued has quadratic complexity.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@64262 91177308-0d34-0410-b5e6-96231b3b80d8