[llvm-mca] Add cycleBegin/cycleEnd callbacks to mca::Stage.

[llvm-mca] Add cycleBegin/cycleEnd callbacks to mca::Stage.

Summary:
This patch  clears up some of the semantics within the Stage class.  Now, preExecute
can be called multiple times per simulated cycle.  Previously preExecute was
only called once per cycle, and postExecute could have been called multiple
times.

Now, cycleStart/cycleEnd are called only once per simulated cycle.
preExecute/postExecute can be called multiple times per cycle.  This
occurs because multiple execution events can occur during a single cycle.

When stages are executed (Pipeline::runCycle), the postExecute hook will
be called only if all Stages return a success from their 'execute' callback.

Reviewers: andreadb, courbet, RKSimon

Reviewed By: andreadb

Subscribers: tschuett, gbedwell, llvm-commits

Differential Revision: https://reviews.llvm.org/D49250

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@336959 91177308-0d34-0410-b5e6-96231b3b80d8