[llvm-mca] Introduce the ExecuteStage (was originally the Scheduler class).

Summary: This patch transforms the Scheduler class into the ExecuteStage.  Most of the logic remains.  

Reviewers: andreadb, RKSimon, courbet

Reviewed By: andreadb

Subscribers: mgorny, javed.absar, tschuett, gbedwell, llvm-commits

Differential Revision: https://reviews.llvm.org/D47246

llvm-svn: 334679