[llvm-mca] Move the AssembleInput logic into its own class.

Summary:
This patch introduces a CodeRegionGenerator class which is responsible for parsing some type of input and creating a 'CodeRegions' instance for use by llvm-mca.  In the future, we will also have a CodeRegionGenerator subclass for converting an input object file into CodeRegions.  For now, we only have the subclass for converting input assembly into CodeRegions.

This is mostly a NFC patch, as the logic remains close to the original, but now encapsulated in its own class and moved outside of llvm-mca.cpp.

Reviewers: andreadb, courbet, RKSimon

Reviewed By: andreadb

Subscribers: mgorny, tschuett, gbedwell, llvm-commits

Differential Revision: https://reviews.llvm.org/D54179

llvm-svn: 346344