[PATCH] Remove unnecessary "& 1" in year_month_day_last::day()

[Cassio Neri <cassio.neri@gmail.com>]

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When year_month_day_last::day() was implemented, Dr. Matthias Kretz realised
that the operation "& 1" wasn't necessary but we did not patch it at that
time. This patch removes the unnecessary operation.

libstdc++-v3/ChangeLog:

* include/std/chrono:

diff --git a/libstdc++-v3/include/std/chrono
b/libstdc++-v3/include/std/chrono
index 10e868e5a03..c979a5d05dd 100644
--- a/libstdc++-v3/include/std/chrono
+++ b/libstdc++-v3/include/std/chrono
@@ -1800,8 +1800,10 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
       {
  const auto __m = static_cast<unsigned>(month());

- // Excluding February, the last day of month __m is either 30 or 31 or,
- // in another words, it is 30 + b = 30 | b, where b is in {0, 1}.
+ // Assume 1 <= __m <= 12, otherwise month().ok() == false and the result
+ // of day() is unspecified. Excluding February, the last day of month __m
+ // m is either 30 or 31 or, in another words, it is 30 | b, where b is in
+ // {0, 1}.

  // If __m in {1, 3, 4, 5, 6, 7}, then b is 1 if, and only if __m is odd.
  // Hence, b = __m & 1 = (__m ^ 0) & 1.
@@ -1812,10 +1814,14 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
  // Therefore, b = (__m ^ c) & 1, where c = 0, if __m < 8, or c = 1 if
  // __m >= 8, that is, c = __m >> 3.

+ // Since 30 = (11110)_2 and __m <= 31 = (11111)_2, we have:
+ // 30 | ((__m ^ c) & 1) == 30 | (__m ^ c), that is, the "& 1" is
+ // unnecessary.
+
  // The above mathematically justifies this implementation whose
  // performance does not depend on look-up tables being on the L1 cache.
- return chrono::day{__m != 2 ? ((__m ^ (__m >> 3)) & 1) | 30
-    : _M_y.is_leap() ? 29 : 28};
+ return chrono::day{__m != 2 ? (__m ^ (__m >> 3)) | 30
+ : _M_y.is_leap() ? 29 : 28};
       }

       constexpr