Better annotate unlikely conditions

And in case of gcc we win also a small
speed optimization due to better branch
prediction.

No functional change.

src/types.h

@@ -71,6 +71,14 @@
 #  define FORCE_INLINE  inline
 #endif
 
+#ifdef __GNUC__
+#  define likely(x)   __builtin_expect(!!(x), 1)
+#  define unlikely(x) __builtin_expect(!!(x), 0)
+#else
+#  define likely(x)   (x)
+#  define unlikely(x) (x)
+#endif
+
 #if defined(USE_POPCNT)
 const bool HasPopCnt = true;
 #else