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

[Jay Foad via llvm-commits]

jayfoad wrote:

This is one attempt at preventing s_wit_event from being scheduled too early, which can be bad for performance.

Another approach might be to teach the scheduler to always schedule it as late as possible, but I'm not sure how to implement that.

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