[clang] [llvm] [AMDGPU]: Add and codegen sched_group_barrier_inst (PR #78775)

[Matt Arsenault via cfe-commits]

https://github.com/arsenm commented:

I don't really understand all of these manual scheduling control intrinsic. This brings the total up to 4? I think effort would be better spent making the scheduler better instead of giving users more footguns to shoot themselves (and the compiler) with

https://github.com/llvm/llvm-project/pull/78775