Optimize RNG use in std::sample selection sampling

Optimize RNG use in std::sample selection sampling

2016-10-21  Eelis van der Weegen  <eelis@eelis.net>

	* include/bits/stl_algo.h (__gen_two_uniform_ints): Move logic out
	of shuffle into new function.
	(shuffle): Call __gen_two_uniform_ints.
	(__sample<ForwardIterator, OutputIterator, Cat, Size, URBG>): Use
	__gen_two_uniform_ints and perform two samples at a time.

From-SVN: r241414