Add unreachable statement. No functional change.

Idea by Ronald de Man (https://groups.google.com/d/msg/fishcooking/7jgBrUXX0cs/GK8l36yGM7oJ) ; compiled and tested with gcc/icc/clang/intel compiler.
Here are the bench results showing small (~0.6%) speedup for gcc/icc ; msvc bench not included as they are in a virtual machine.
I know this is too much lines, but I have no idea on how to reduce this ;)

master (gcc 4.8.1) |  unreachable (gcc 4.8.1) 
------------|-----------------
2280255 | 2288071
2284457 | 2289882
2276069 | 2281454
2270710 | 2289278
2275472 | 2285058
2282654 | 2289278

master (clang 3.3) |  unreachable (clang 3.3)
------------|-----------------
2224725 | 2222446
2223585 | 2220740
2223585 | 2222446
2223015 | 2221308
2225295 | 2221308
2224725 | 2221308

master (intel compiler 14.0.2) | unreachable (intel compiler 14.0.2)
------------|-----------------
2186609 | 2192686
2177282 | 2191579
2182211 | 2191579
2184408 | 2193240
2179471 | 2191026
2182211 | 2181662