MachineSink: Sort the successors before trying to find SuccToSinkTo.

MachineSink: Sort the successors before trying to find SuccToSinkTo.

One motivating example is to sink an instruction from a basic block which has
two successors: one outside the loop, the other inside the loop. We should try
to sink the instruction outside the loop.

rdar://11980766

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@161062 91177308-0d34-0410-b5e6-96231b3b80d8