Improve AVX512 testcase
Improve AVX512 testcase

clang 3.4 managed to accept the original test code, only to fail on the actual Skylake asm later
xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS,xianyi/OpenBLAS