[GPU] Add barriers when resolving GPUMappedForall to fix race condition

[nirvedhmeshram]

The barriers added here can be pessimistic and we can look into optimizing them at a later point if needed. However, we end up with a race if we dont have them.
In some local testing I did on a MI300 GPU, I did not find any significant performance impact by these barriers. For example an unaligned matmul + elementwise took 47us and 48us with and without the barriers respectively with TileAndFuse with padding support and the corresponding default path takes 68us. The prefill stage of ToyLLAMA took 325us and 324us respectively with and without barriers while the default path takes 461us.