time: In the Time_Before method, cache the result of sec() calls.

The current implementation calls sec() for both the receiver t and
the parameter u twice, if the seconds of both times are equal.
This is the same optimization that has already been implemented
in the Time_After method.

This fixes no issue, as it is a trivial change.

src/time/time.go

@@ -252,7 +252,9 @@ func (t Time) Before(u Time) bool {
  if t.wall&u.wall&hasMonotonic != 0 {
  return t.ext < u.ext
  }
- return t.sec() < u.sec() || t.sec() == u.sec() && t.nsec() < u.nsec()
+ ts := t.sec()
+ us := u.sec()
+ return ts < us || ts == us && t.nsec() < u.nsec()
 }
 
 // Equal reports whether t and u represent the same time instant.