[llvm-mca] Move DispatchStage::cycleEvent to preExecute. NFC.

Summary:
This is an intermediate change, it moves the non-notification logic from
Backend::notifyCycleBegin to runCycle().
    
Once the scheduler becomes part of the Execution stage
the explicit call to Scheduler::cycleEvent will disappear.
    
The logic for Dispatch::cycleEvent() can be in
the preExecute phase, which this patch addresses.


Reviewers: andreadb, RKSimon, courbet

Reviewed By: andreadb

Subscribers: tschuett, gbedwell, llvm-commits

Differential Revision: https://reviews.llvm.org/D47213

llvm-svn: 333029