Add likely/unlikely() branch hint macros.

These are useful for very hot code paths. Because it's easy to guess
wrongly about likelihood, and because such likelihoods change over time,
they should be used sparingly.

Past tests have shown it'd be a good idea to use them in some places,
e.g. in error checks around ereports that ERROR out, but that's work for
later.

Discussion: <20160727004333.r3e2k2y6fvk2ntup@alap3.anarazel.de>

src/include/c.h

@@ -939,6 +939,22 @@ typedef NameData *Name;
 #endif
 
 
+/*
+ * Hints to the compiler about the likelihood of a branch. Both likely() and
+ * unlikely() return the boolean value of the contained expression.
+ *
+ * These should only be used sparingly, in very hot code paths. It's very easy
+ * to mis-estimate likelihoods.
+ */
+#if __GNUC__ >= 3
+#define likely(x)	__builtin_expect((x) != 0, 1)
+#define unlikely(x) __builtin_expect((x) != 0, 0)
+#else
+#define likely(x)	((x) != 0)
+#define unlikely(x) ((x) != 0)
+#endif
+
+
 /* ----------------------------------------------------------------
  *				Section 8:	random stuff
  * ----------------------------------------------------------------