libstdc++: std::includes performance tweak

libstdc++: std::includes performance tweak

A small tweak to the implementation of __includes, which in my
application saves 20% of the running time. I noticed it because using
range-v3 was giving unexpected performance gains.

Some of the gain comes from pulling the 2 calls ++__first1 out of the
condition so there is just one call. And most of the gain comes from
replacing the resulting

if (__comp(__first1, __first2))
  ;
else
  ++__first2;

with

if (!__comp(__first1, __first2))
  ++__first2;

I was very surprised that the code ended up being so different for such
a change, and I still don't really understand where the extra time is
going...

Anyway, while I blame the compiler for not generating very good code
with the current implementation, I believe the change can be seen as a
simplification.

libstdc++-v3/ChangeLog:

	* include/bits/stl_algo.h (__includes): Simplify the code.