libstdc++: Remove unnecessary tag dispatching in std::vector

libstdc++: Remove unnecessary tag dispatching in std::vector

There's no need to call a _M_xxx_dispatch function with a
statically-known __false_type tag, we can just directly call the
function that should be dispatched to. This will compile a tiny bit
faster and save a function call with optimization or inlining turned
off.

Also add the always_inline attribute to the __iterator_category helper
used for dispatching on the iterator category.

libstdc++-v3/ChangeLog:

	* include/bits/stl_iterator_base_types.h (__iterator_category):
	Add always_inline attribute.
	* include/bits/stl_vector.h (assign(Iter, Iter)): Call
	_M_assign_aux directly, instead of _M_assign_dispatch.
	(insert(const_iterator, Iter, Iter)): Call _M_range_insert
	directly instead of _M_insert_dispatch.