libstdc++: Enhance branching in std::inplace_merge and std::stable_sort

libstdc++: Enhance branching in std::inplace_merge and std::stable_sort

When we manage to allocate a buffer of the expected size we can simplify the code to
perform the expected algorithm.

libstdc++-v3/ChangeLog:

	* include/bits/stl_algo.h
	(__merge_adaptive): Adapt to merge only when buffer is large enough..
	(__merge_adaptive_resize): New, adapt merge when buffer is too small.
	(__inplace_merge): Adapt, use latter.
	(__stable_sort_adaptive): Adapt to sort only when buffer is large enough.
	(__stable_sort_adaptive_resize): New, adapt sort when buffer is too small.
	(__stable_sort): Adapt, use latter.