[Bug libstdc++/83938] Speed up inplace_merge() algorithm & fix inefficient logic

[john.chang at samba dot tv]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83938

--- Comment #1 from WeiChungChang <john.chang at samba dot tv> ---
Created attachment 43182
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=43182&action=edit
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