[llvm-mca] Move the RegisterFile class into its own translation unit. NFC

Summary: This change will help us turn the DispatchUnit into its own stage.

Reviewers: andreadb, RKSimon, courbet

Reviewed By: andreadb, courbet

Subscribers: mgorny, tschuett, gbedwell, llvm-commits

Differential Revision: https://reviews.llvm.org/D46916

llvm-svn: 332493