[llvm-mca] Add method cycleEvent() to class Scheduler. NFCI

The goal of this patch is to simplify the Scheduler's interface in preparation
for D50929.
Some methods in the Scheduler's interface should not be exposed to external
users, since their presence makes it hard to both understand, and extend the
Scheduler's interface.

This patch removes the following two methods from the public Scheduler's API:
 - reclaimSimulatedResources()
 - updatePendingQueue()
Their logic has been migrated to a new method named 'cycleEvent()'.

Methods 'updateIssuedSet()' and 'promoteToReadySet()' still exist. However,
they are now private members of class Scheduler.

This simplifies the interaction with the Scheduler from the ExecuteStage.

llvm-svn: 340273