[AMDGPU][SIInsertWaitcnt][NFC] Access Waitcnt elements using InstCounterType (#178345)
This patch introduces `get(T)` and `set(T, Val)` functions for Waitcnt
and removes getCounterRef() and getWait(). For this to work we also need
to move InstrCounterType to AMDGPUBaseInfo.h.
Please note that the member variables are still public to keep this
patch small.
They will be replaced in the follow-up patch.