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

[Jay Foad via llvm-commits]

jayfoad wrote:

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

Agreed, and we need a comment explaining that it's used for read and write pre-GFX10 but only for read in GFX10+.

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