Fix BOF in the last channel handling logic of SSE ShuffleChannel

[junwha]

Fix https://github.com/Tencent/ncnn/issues/5734

• apply SSE logic until size-1
• apply linear logic for the last n elements

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 90.03%. Comparing base (1c7af00) to head (b2ee3aa).

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HEAD has 32 uploads less than BASE

Flag	BASE (1c7af00)	HEAD (b2ee3aa)
	33	1

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #5735       +/-   ##
===========================================
- Coverage   95.15%   90.03%    -5.13%     
===========================================
  Files         793      265      -528     
  Lines      270315    77624   -192691     
===========================================
- Hits       257218    69886   -187332     
+ Misses      13097     7738     -5359     

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[junwha]

Hi @nihui, Thank you for the fast response and review! I will reflect the unrolling part.

Could I ask why do you think the out-of-bound risk does not exist in pack8/16 codes? As far as I think, it also accesses (start of the channel) + (size-1)*(element size) + (element size)/2 in the same manner, which is beyond the size of the channel.

Thank you for your effort on this issue!