Fix ABC FOLD rule with constants.

Reported by XmiliaH.

src/lj_opt_fold.c

@@ -1682,14 +1682,15 @@ LJFOLDF(abc_fwd)
 LJFOLD(ABC any KINT)
 LJFOLDF(abc_k)
 {
+  PHIBARRIER(fleft);
   if (LJ_LIKELY(J->flags & JIT_F_OPT_ABC)) {
     IRRef ref = J->chain[IR_ABC];
     IRRef asize = fins->op1;
     while (ref > asize) {
       IRIns *ir = IR(ref);
       if (ir->op1 == asize && irref_isk(ir->op2)) {
-	int32_t k = IR(ir->op2)->i;
-	if (fright->i > k)
+	uint32_t k = (uint32_t)IR(ir->op2)->i;
+	if ((uint32_t)fright->i > k)
 	  ir->op2 = fins->op2;
 	return DROPFOLD;
       }