Perl_regexec_flags - create and populate a new SV in one call, not two.

Perl_regexec_flags had these lines to first create a new SV, then assign
to it the value(s) of an existing SV:
```
        reginfo->sv = newSV_type(SVt_NULL);
        SvSetSV_nosteal(reginfo->sv, sv);
```
This is two calls into _sv.c_ and, if the existing SV is `SvOK`, will
incur an SV upgrade in the process. Those lines are preceded with the
comment:
`Not newSVsv, either, as it does not COW.`

However, the underpinnings of `newSVsv` and variants do support COW
nowadays, so we can now just do:
```
reginfo->sv = newSVsv_flags(sv, SV_GMAGIC|SV_NOSTEAL|SV_DO_COW_SVSETSV);
```

Author: richardleach

regexec.c

@@ -3863,10 +3863,8 @@ Perl_regexec_flags(pTHX_ REGEXP * const rx, char *stringarg, char *strend,
            S_cleanup_regmatch_info_aux has executed (registered by
            SAVEDESTRUCTOR_X below).  S_cleanup_regmatch_info_aux modifies
            magic belonging to this SV.
-           Not newSVsv, either, as it does not COW.
         */
-        reginfo->sv = newSV_type(SVt_NULL);
-        SvSetSV_nosteal(reginfo->sv, sv);
+        reginfo->sv = newSVsv_flags(sv, SV_GMAGIC|SV_NOSTEAL|SV_DO_COW_SVSETSV);
         SAVEFREESV(reginfo->sv);
     }