Replace JMPs in Bgotocode with JMP SHORTs whereever possible. This routine depends on FLcode jumps to only be forward referenced. BFLjmpoptdone is set to true if nothing more can be done with this block.
extern(C++) int branch (
.backend .cc .block* bl,
) nothrow @trusted;
flag !=0 means don't have correct Boffsets yet
number of bytes saved