[MachineCombiner] Work with itineraries

MachineCombiner predicated its use of scheduling-based metrics on
hasInstrSchedModel(), but useful conclusions can be drawn from pipeline
itineraries as well. Almost all of the logic (except for resource tracking in
preservesResourceLen) can be used if we have an itinerary, so enable it in that
case as well.

This will be used by the PowerPC backend in an upcoming commit.

llvm-svn: 242277