Cleaning 'luaP_isIT' and 'luaP_isOT'

- 'luaP_isOT' is only used for tests, so it is defined as a macro to avoid
wasting space with an unused function.
- 'luaP_isIT' must include OP_VARARGPREP.

lcode.c

@@ -1934,8 +1934,6 @@ void luaK_finish (FuncState *fs) {
     p->flag &= cast_byte(~PF_VAHID);  /* then it will not use hidden args. */
   for (i = 0; i < fs->pc; i++) {
     Instruction *pc = &p->code[i];
-    /* avoid "not used" warnings when assert is off (for 'onelua.c') */
-    (void)luaP_isOT; (void)luaP_isIT;
     lua_assert(i == 0 || luaP_isOT(*(pc - 1)) == luaP_isIT(*pc));
     switch (GET_OPCODE(*pc)) {
       case OP_RETURN0: case OP_RETURN1: {