x86 avx512 optimization

[nihui]

No description provided.

[codecov-commenter]

Codecov Report

Merging #3691 (0a2ffaa) into master (9298d05) will decrease coverage by 1.38%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3691      +/-   ##
==========================================
- Coverage   93.14%   91.76%   -1.39%     
==========================================
  Files         647      620      -27     
  Lines      167439   157831    -9608     
==========================================
- Hits       155964   144835   -11129     
- Misses      11475    12996    +1521     

Impacted Files	Coverage Δ
src/layer/x86/convolution_x86.cpp	96.97% <ø> (-1.00%)	⬇️
src/layer/x86/eltwise_x86.cpp	99.56% <100.00%> (-0.15%)	⬇️
src/layer/x86/convolution_3x3_pack4.h	0.00% <0.00%> (-88.11%)	⬇️
...c/layer/x86/convolution_winograd_transform_pack4.h	29.39% <0.00%> (-69.65%)	⬇️
src/layer/x86/convolution_3x3_pack1to4.h	49.33% <0.00%> (-50.67%)	⬇️
src/layer/x86/quantize_x86.cpp	69.21% <0.00%> (-27.72%)	⬇️
src/layer/x86/x86_usability.h	83.43% <0.00%> (-16.57%)	⬇️
src/command.cpp	74.60% <0.00%> (-13.05%)	⬇️
src/layer/x86/concat_x86.cpp	86.98% <0.00%> (-11.31%)	⬇️
src/layer/x86/slice_x86.cpp	87.95% <0.00%> (-10.77%)	⬇️
... and 148 more

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[lgtm-com]

This pull request introduces 1 alert when merging 4f0eacb into 9298d05 - view on LGTM.com

new alerts:

• 1 for Comparison result is always the same