[MCSched] Bind PFM Counters to the CPUs instead of the SchedModel.

Summary:
The pfm counters are now in the ExegesisTarget rather than the
MCSchedModel (PR39165).

This also compresses the pfm counter tables (PR37068).

Reviewers: RKSimon, gchatelet

Subscribers: mgrang, llvm-commits

Differential Revision: https://reviews.llvm.org/D52932

llvm-svn: 345243