Factor out the SchedRemainder/SchedBoundary from GenericScheduler strategy.

These helper classes take care of the book-keeping the drives the
GenericScheduler heuristics. It is likely that developers writing
target-specific schedulers that work similarly to GenericScheduler
will want to use these helpers too. The immediate goal is to develop a
GenericPostScheduler that can run in place of the old PostRAScheduler,
but will use the new machine model.

No functionality change intended.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196643 91177308-0d34-0410-b5e6-96231b3b80d8