[llvm] [AMDGPU] Make S_WAIT_EVENT a scheduling boundary (PR #129032)

[Jay Foad via llvm-commits]

jayfoad wrote:

@kerbowa your suggestion to use `sched_barrier(0)` in the front end seems to be working well for our use case, so I may abandon this patch.

https://github.com/llvm/llvm-project/pull/129032