[MachinePipeliner] Separate schedule emission, NFC

This is the first stage in refactoring the pipeliner and making it more
accessible for backends to override and control. This separates the logic and
state required to *emit* a scheudule from the logic that *computes* and
validates a schedule.

This will enable (a) new schedule emitters and (b) new modulo scheduling
implementations to coexist.

NFC.

Differential Revision: https://reviews.llvm.org/D67006

llvm-svn: 370500