[libcxx] span: Remove unneeded comparison

[miscco]

Summary

size_t is always greater than 0 so remove the artifact from the old index_type

[miscco]

Do not change the comment...

[miscco]

Ping trivially correct change

[ldionne]

Thanks for the patch!

libcxx/include/span

//  ~span() noexcept = default;
     }
 
     _LIBCPP_INLINE_VISIBILITY constexpr size_type size()       const noexcept { return _Extent; }
     _LIBCPP_INLINE_VISIBILITY constexpr size_type size_bytes() const noexcept { return _Extent * sizeof(element_type); }
     _LIBCPP_INLINE_VISIBILITY constexpr bool empty()           const noexcept { return _Extent == 0; }
 
     _LIBCPP_INLINE_VISIBILITY constexpr reference operator[](size_type __idx) const noexcept
     {
-        _LIBCPP_ASSERT(__idx >= 0 && __idx < size(), "span<T,N>[] index out of bounds");
+        _LIBCPP_ASSERT(__idx < size(), "span<T,N>[] index out of bounds");
         return __data[__idx];
     }
 
     _LIBCPP_INLINE_VISIBILITY constexpr reference front() const noexcept
     {
         static_assert(_Extent > 0, "span<T,N>[].front() on empty span");
         return __data[0];
     }
//    ~span() noexcept = default;
     }
 
     _LIBCPP_INLINE_VISIBILITY constexpr size_type size()       const noexcept { return __size; }
     _LIBCPP_INLINE_VISIBILITY constexpr size_type size_bytes() const noexcept { return __size * sizeof(element_type); }
     _LIBCPP_INLINE_VISIBILITY constexpr bool empty()           const noexcept { return __size == 0; }
 
     _LIBCPP_INLINE_VISIBILITY constexpr reference operator[](size_type __idx) const noexcept
     {
-        _LIBCPP_ASSERT(__idx >= 0 && __idx < size(), "span<T>[] index out of bounds");
+        _LIBCPP_ASSERT(__idx < size(), "span<T>[] index out of bounds");
         return __data[__idx];
     }
 
     _LIBCPP_INLINE_VISIBILITY constexpr reference front() const noexcept
     {
         _LIBCPP_ASSERT(!empty(), "span<T>[].front() on empty span");
         return __data[0];
     }