[PATCH] libstdc++: More efficient std::chrono::year::leap.

[Cassio Neri via Gcc-patches]

Simple change to std::chrono::year::is_leap. If a year is multiple of 100,
then it's divisible by 400 if and only if it's divisible by 16. The latter
allows for better code generation.

Tested on x86_64-pc-linux-gnu.

libstdc++-v3/ChangeLog:

* include/std/chrono:

diff --git a/libstdc++-v3/include/std/chrono b/libstdc++-v3/include/std/chrono
index 4631a727d73..6221d02c1b3 100644
--- a/libstdc++-v3/include/std/chrono
+++ b/libstdc++-v3/include/std/chrono
@@ -1612,7 +1612,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
  constexpr uint32_t __offset       = __max_dividend / 2 / 100 * 100;
  const bool __is_multiple_of_100
   = __multiplier * (_M_y + __offset) < __bound;
- return (!__is_multiple_of_100 || _M_y % 400 == 0) && _M_y % 4 == 0;
+ return (!__is_multiple_of_100 || _M_y % 16 == 0) && _M_y % 4 == 0;
       }

       explicit constexpr