luaK_codeAsBx
j = luaK_codeAsBx(fs, OP_JMP, 0, NO_JUMP);
prep = isnum ? luaK_codeAsBx(fs, OP_FORPREP, base, NO_JUMP) : luaK_jump(fs);
endfor = luaK_codeAsBx(fs, OP_FORLOOP, base, NO_JUMP);
endfor = luaK_codeAsBx(fs, OP_TFORLOOP, base + 2, NO_JUMP);