likely/unlikely equivalent for MSVC

C++20 standard will include [[likely]] and [[unlikely]] branch prediction attributes.

The latest revision of attribute proposal can be found from http://wg21.link/p0479

The original attribute proposal can be found from http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0479r0.html

Programmers should prefer PGO. Attributes can easily reduce performance if applied incorrectly or they later become incorrect when program changes.

According to http://www.akkadia.org/drepper/cpumemory.pdf (page 57), it still makes sense to use static branch prediction even if CPU predicts correctly dynamically. The reason for that is that L1i cache will be used even more efficiently if static prediction was done right.