[MachineOutliner] Only outline candidates of length >= 2

[MachineOutliner] Only outline candidates of length >= 2

Since we don't factor in instruction lengths into outlining calculations
right now, it's never the case that a candidate could have length < 2.

Thus, we should quit early when we see such candidates.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@310894 91177308-0d34-0410-b5e6-96231b3b80d8