sdpa_with_mask CPU dispatch count regressed to 2 after iree-org/iree#24127

[IanWood1]

The CPU-STATS-CHECK: "dispatch-count" in tests/lit/test_sdpa_asm_emitter_with_mask.cpp was bumped from 1 to 2 in the 3.12.0rc20260416 bump (#345).

The regression is caused by iree-org/iree#24127, which puts the previously-fused sdpa-with-mask into two dispatches on the CPU backend. AMDGPU is unaffected (still 1 dispatch).

[IanWood1]

AMDGPU still fuses to a single dispatch because at --iree-opt-level=O3 (or O2) aggressive fusion is enable in FormDispatchRegions. CPU uses the default optimization level O0.