It turns out that the __builtin_amdgcn_s_barrier() alone does not emit
a fence. We somehow got away with this and assumed it would work as it
(hopefully) is correct on the NVIDIA path where we just emit a
__syncthreads. After talking to @arsenm we now (mostly) align with the
OpenCL barrier implementation [1] and emit explicit fences for AMDGPUs.
It seems this was the underlying cause for #59759, but I am not 100%
certain. There is a chance this simply hides the problem.
Fixes: https://github.com/llvm/llvm-project/issues/59759
[1] 07b347366e/opencl/src/workgroup/wgbarrier.cl (L21)
67fed132f3