vulkan: Pad N dimension of B matrix for coopmat2 perf, to avoid bounds checking

[jeffbolznv]

This is stacked on #12258 and reserves padding space for the B matrix in the N dimension, so the coopmat2 shader can avoid bounds checking and take the fast path.

Results for Phi-3-mini-4k-instruct-q4.gguf on RTX 4070:

		master		PR12258		PR12273		speedup vs PR12258
							
        pp5	447.15		462.92		466.92		0.86%
       pp10	274.87		494.28		517.04		4.60%
       pp20	564.73		991.1		1045.43		5.48%
       pp31	863.29		1418.82		1730.83		21.99%
       pp32	894.75		1680.77		1745.39		3.84%
       pp33	909.48		1293.95		1512.23		16.87%
       pp48	1347.26		1766.27		2249.4		27.35%
       pp54	1465.89		2030.28		2318.44		14.19%
       pp63	1689.89		2405.95		2694.4		11.99%
       pp64	1702.93		2910.49		2764.43		-5.02%
       pp65	1741.64		1903.68		2260.9		18.76%
       pp80	2039.35		2288.33		2835.27		23.90%
       pp96	2397.05		2722.58		3189.51		17.15%
      pp112	2732.28		3245.6		3632.44		11.92%
      pp113	2787.73		3057.98		3656.96		19.59%
      pp127	3064.93		3449.08		4087.5		18.51%
      pp128	3817.62		4258.3		4174.96		-1.96%
      pp129	2246.45		2516.47		2734.41		8.66%
      pp140	2357.01		2710.37		2907.37		7.27%
      pp160	2606.54		2998.89		3356.15		11.91%
      pp180	2891.38		3292.31		3639.65		10.55%
      pp192	3036.42		3622.36		3857.68		6.50%
      pp200	3106.62		3610.68		3979.56		10.22%
      pp210	3245.74		3771.82		4258.49		12.90%
      pp230	3493.62		4152.18		4589.22		10.53%
      pp248	3678.74		4357.96		4761.21		9.25%
      pp255	3719.19		4338.61		5024.18		15.80%
      pp256	4818.48		5035.21		4919.83		-2.29%
      pp257	2898.17		3605.43		3788.88		5.09%
      pp280	3143.84		3830.24		4150.83		8.37%
      pp300	3300.25		4080.59		4432.56		8.63%
      pp320	3498.11		4217.64		4545.71		7.78%
      pp350	3672.25		4548.51		4877.21		7.23%
      pp384	4251.46		5316.02		5189.39		-2.38%
      pp410	3478.13		4287.22		4580.83		6.85%
      pp448	3741.12		4645.57		4945.34		6.45%
      pp480	3886.56		4830.78		5208.15		7.81%
      pp490	3980.64		4909.99		5185.52		5.61%
      pp511	4051.34		5030.26		5365		6.65%
      pp512	5434.86		5435.16		5398.73		-0.67%
      pp513	4935.44		5077.35		4958.86		-2.33%
      pp767	4633.69		4985.33		5141.08		3.12%
      pp768	5171.77		5251.81		5175.3		-1.46%
      pp769	4143.96		4575.35		4685.68		2.41%
     pp1023	4575.58		5227.51		5300.75		1.40%
     pp1024	5324.02		5318.13		5294.66		-0.44%
     pp1025	5119.71		5118.21		5023.31		-1.85%
     pp2047	4748.43		5093.44		5117.1		0.46%
     pp2048	5152.48		5185.36		5116.32		-1.33%
     pp2049	5045.99		5021.26		5016.38		-0.10%