pp_substr: no need to redo work when cloning the replacement string

Currently, when copying the replacement string in preparation for
`sv_utf8_upgrade()`, `pp_substr` calls:

    repl_sv_copy = newSVsv(repl_sv);

However, `repl_sv` was previously checked for GMAGIC, coerced to a
string if necessary, and the char * and length obtained by:

    repl = SvPV_const(repl_sv, repl_len);

We should be able to produce a copy more directly by just doing:

    repl_sv_copy = newSVpvn(repl, repl_len);

pp.c

@@ -3434,7 +3434,7 @@ PP(pp_substr)
             SV* repl_sv_copy = NULL;
 
             if (repl_need_utf8_upgrade) {
-                repl_sv_copy = newSVsv(repl_sv);
+                repl_sv_copy = newSVpvn(repl, repl_len);
                 sv_utf8_upgrade(repl_sv_copy);
                 repl = SvPV_const(repl_sv_copy, repl_len);
             }