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

[redi at gcc dot gnu.org via Gcc-bugs]

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

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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   Target Milestone|9.4                         |11.0