sycl: fuse UNARY(silu|sigmoid|softplus) + MUL (#26411)
Measured on Arc Pro B70 (Battlemage), Qwen3.6-27B Q4_K_M, -fa on, f16 KV,
-b 2048 -ub 2048, llama-bench -r 3, three interleaved A/B rounds:
pp2048 1014.70 -> 1018.56 t/s (+0.38%, within run-to-run spread)
tg128 23.73 -> 23.86 t/s (+0.57%)
tg128 @ d4096 22.71 -> 22.86 t/s (+0.62%)
