[llvm] [AMDGPU][InsertWaitCnts][NFC] Merge VMEM_ACCESS and VMEM_READ_ACCESS into a single event type (PR #171973)

[Matt Arsenault via llvm-commits]

https://github.com/arsenm commented:

If they are going to cover read and write, this should merge to the VMEM_ACCESS name? 

https://github.com/llvm/llvm-project/pull/171973