[llvm-mca] Updates comment in code, and remove some stale comments. NFC

Also, rename fields `TotalMappings` and `NumUsedMappings` in struct
RegisterMappingTracker into `NumPhysRegs` and `NumUsedPhysRegs`.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@335219 91177308-0d34-0410-b5e6-96231b3b80d8
4 files changed