instancenorm x86 simd

[futz12]

No description provided.

[tencent-adm]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you all sign our Contributor License Agreement before we can accept your contribution.
1 out of 2 committers have signed the CLA.

✅ nihui
❌ futz12
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[github-actions]

The binary size change of libncnn.so (bytes)

architecture	base size	pr size	difference
x86_64	16488320	16530608	+42288 ⚠️
armhf	7368864	7369820	+956 ⚠️
aarch64	10779000	10775560	-3440 😘

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.37%. Comparing base (73d8500) to head (b44c4b4).
Report is 22 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6097      +/-   ##
==========================================
- Coverage   95.59%   95.37%   -0.23%     
==========================================
  Files         827      775      -52     
  Lines      270116   219075   -51041     
==========================================
- Hits       258226   208932   -49294     
+ Misses      11890    10143    -1747     

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[nihui]

please for (; i + 15 < size; i += 16)

[nihui]

Thanks for your contribution !