[llvm-mca] Turn InstructionTables into a Stage.

Summary:
This patch converts the InstructionTables class into a subclass of mca::Stage.  This change allows us to use the Stage's inherited Listeners for event notifications.  This also allows us to create a simple pipeline for viewing the InstructionTables report.

I have been working on a follow on patch that should cleanup addView in InstructionTables.  Right now, addView adds the view to both the Listener list and Views list.  The follow-on patch addresses the fact that we don't really need two lists in this case.  That change is not specific to just InstructionTables, so it will be a separate patch. 

Reviewers: andreadb, courbet, RKSimon

Reviewed By: andreadb

Subscribers: tschuett, gbedwell, llvm-commits

Differential Revision: https://reviews.llvm.org/D49329

llvm-svn: 337113