In bidirectional scheduling this gives more stable results than just
comparing the "reason" fields of the top/bottom node because the reason
field may be higher depending on what other nodes are in the queue.
Differential Revision: http://reviews.llvm.org/D19401
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@273755 91177308-0d34-0410-b5e6-96231b3b80d8