Proposal: An analogue to gcc's __builtin_expect

[mrakh]

In GNU C, one can use the __builtin_expect construct in a conditional statement as a hint to the compiler, where the resulting assembly is arranged in a way that reduces the expected number of jumps and thus minimizes CPU pipeline stalls. It's often used in hot code where branching is unavoidable. I believe that exposing similar functionality to Zig would be useful, either as a built-in, or as a qualifier keyword used with conditional flow constructs (e.g. if, while).

[rohlem]

duplicate of #489