[llvm-mca] Lift the logic of the RetireControlUnit from the Dispatch...

[llvm-mca] Lift the logic of the RetireControlUnit from the Dispatch translation unit into its own translation unit. NFC

The logic remains the same.  Eventually, I see the RCU acting as its own separate stage in the instruction pipeline.

Differential Revision: https://reviews.llvm.org/D46331

llvm-svn: 331316