likely/unlikely macros

[Dan Eble]

http://stackoverflow.com/questions/109710/likely-unlikely-macros-in-the-linux-kernel-how-do-they-work-whats-their

> #define likely(x) __builtin_expect((x),1)
> #define unlikely(x) __builtin_expect((x),0)

Are there any objections to adding something like these to Lilypond?  (I’d use 
static_cast<bool>(x) though.)

An obvious place to use these is around the predicate in LY_ASSERT_TYPE where 
failure is unlikely.  I’m sure you can think of several more.
— 
Dan