[AMDGPU][SIInsertWaitcnts][NFC] Introduce Counter class
This is the first patch in a series of patches that encapsulate the score
counters (and all related logic) in a class.
The goal is to provide a high level API and not provide raw access to the
LB and UB values. But until all functionality has been migrated to class
member functions, we will still expose these values through getters/setters.