[SchedModel] Complete models shouldn't match against itineraries when they don't use them (PR35639)

For schedule models that don't use itineraries, checkCompleteness still checks that an instruction has a matching itinerary instead of skipping and going straight to matching the InstRWs. That doesn't seem to match what happens in TargetSchedule.cpp

This patch causes problems for a number of models that had been incorrectly flagged as complete.

Differential Revision: https://reviews.llvm.org/D43235

llvm-svn: 329280