Add missing PHI barrier to string.sub(str, a, b) == kstr FOLD rule.

src/lj_opt_fold.c

@@ -1819,6 +1819,7 @@ LJFOLDF(merge_eqne_snew_kgc)
 #define FOLD_SNEW_TYPE8		IRT_U8  /* Prefer unsigned loads. */
 #endif
 
+  PHIBARRIER(fleft);
   if (len <= FOLD_SNEW_MAX_LEN) {
     IROp op = (IROp)fins->o;
     IRRef strref = fleft->op1;