libstdc++: Improve and cleanup mdspan related code.

libstdc++: Improve and cleanup mdspan related code.

The improvement is that in __index_type_cast, we don't need to check at
runtime if we know that _IndexType is smaller than _OIndexType.

The cleanup is whitespace (overlength lines) in <mdspan>, grouping is_always_foo
and is_foo together, and de-uglifying a variable in test code.

libstdc++-v3/ChangeLog:

	* include/std/mdspan (__mdspan::__index_type_cast): Optimize by
	skipping a __glibcxx_assert if it's know at compile-time.
	(std::layout_left_padded, std::layout_righ_padded): Reorder
	is_always_strided and is_unique member functions.
	* testsuite/23_containers/mdspan/int_like.h: Rename _M_i to
	value.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>
Signed-off-by: Luc Grosheintz <luc.grosheintz@gmail.com>