pgsql: Add likely/unlikely() branch hint macros.

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(dot)r3e2k2y6fvk2ntup(at)alap3(dot)anarazel(dot)de>

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/aa3ca5e3dd60bf0b992b74f955378f28e601292a

Modified Files
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src/include/c.h | 16 ++++++++++++++++
1 file changed, 16 insertions(+)