sycl: coalesce the ssm_conv window loads (#26612)
test-backend-ops perf -o SSM_CONV on an Arc Pro B70, interleaved A/B against
master, 6 reps, us/run:
ne_a=[515,3328,1,1] ne_b=[4,3328,1,1] n_t=512 97.68 -> 52.95 1.85x
ne_a=[937,8192,1,1] ne_b=[4,8192,1,1] n_t=934 516.16 -> 276.13 1.87x
ne_a=[4,3328,1,1] ne_b=[4,3328,1,1] n_t=1 2.73 -> 2.71 flat
llama-bench on qwen35 27B Q4_K - Medium (48 of its 64 blocks run ssm_conv),
-ngl 99 -fa 1 -ctk f16 -ctv f16, interleaved passes of r=3:
-b 2048 -ub 2048 pp2048 1045.1 / 1043.5 / 1043.7 -> 1069.5 / 1066.3 / 1065.9 +2.2%
-b 2048 -ub 512 pp2048 771.8 / 772.7 -> 785.5 / 786.6 +1.8%
-b 2048 -ub 512 tg128 23.81 / 23.88 -> 23.87 / 23.86 flat
