[BENCHMARK] Regression on Flash Attention

[whitneywhtsang]

Started to fail in https://github.com/intel/intel-xpu-backend-for-triton/actions/runs/16040332329.

Failure is caused by 244c285.

[chengjunlu]

The root cause of this issue is because the new block store lowering fixed some issue that supports boundary check attribute in tl.store.

But the flash attention implementation missed the boundary_check param in the kerne. The segment fault because the kernel instance access the out of bound memory with out boundary check.

Fix the flash attn issue first.