Revert "Perl_op_convert_list - only short circuit CONST OPs with an IsCOW SV"

This reverts commit 80d3e79.

That commit was always intended as a stopgap to be replaced by
b1f270f.

The history behind this was as follows:
* GH#22116 - a902d92 - short-circuited constant folding on CONST OPs,
as this should be unnecessary. However, Dave Mitchell noticed that it
had the inadvertent effect of disabling COW on SVs holding UTF8 string
literals (e.g. `"\x{100}abcd"`).
* b1f270f always seemed like the
best fix, but given the apparent proximity to the 5.42 release date
that commit seemed to be too big a change.
* GH#23296 brought in the now-reverted commit as a stop gap that
retained some of the older, now-unnecessary behaviour.

Author: richardleach

op.c

@@ -5582,7 +5582,7 @@ Perl_op_convert_list(pTHX_ I32 type, I32 flags, OP *o)
             flags |= OPf_SPECIAL;
     }
     if (type == OP_STRINGIFY && OP_TYPE_IS(o, OP_CONST) &&
-        !(flags & OPf_FOLDED) && SvIsCOW(cSVOPx_sv(o)) ){
+        !(flags & OPf_FOLDED) ) {
         assert(!OpSIBLING(o));
         /* Don't wrap a single CONST in a list, process that list,
          * then constant fold the list back to the starting OP.