<span>: checked iterators became too intolerant

Describe the bug
Checked iterators controlled by _ITERATOR_DEBUG_LEVEL provide some safe guarantee, but these assertions became more and more intolerant for programming in normal styles.

Command-line test case

std::array<std::byte, 10> a{};
std::span<std::byte> s1 = a;
std::span<std::byte> s2 = s1.subspan(1);
std::cout << s2.begin() - s1.begin(); // assertion failed while _ITERATOR_DEBUG_LEVEL >= 1

Expected behavior
Both these two span s1 and s2 came from a, hence they share the same underlying sequence.
I think this design makes iterators of the same type pointing to the same element not equivalent. (may be an LWG issue?)
From a programmer’s point of view, the code snippet above is a common requirement, especially when parsing certain data.

STL version

Microsoft Visual Studio Community 2019
Version 16.7.7

Reproduce the command-line case with _ITERATOR_DEBUG_LEVEL >= 1, focusing on std::span::begin() and checked-iterator subtraction. The issue is done when this same-sequence operation no longer asserts, with behavior covered by a regression test; no file or test path is named in the report.