Issue #19350: Revert op.c change from 2a98b8c

This reverts the op.c change from
"newSVpvn_flags().. is more efficient than sv_2mortal(newSVpvn(..))"
full commit id 2a98b8c

For some reason the op.c change tickles a compiler bug. Various minor
changes to the text result in this working as expected, and then other
changes make it fail, etc. I was not able to work out why, the code as
presented is not wrong, although it hides some stuff underneath.

The expression is:

    newSVpvn_flags( PadnamePV(name)+1,PadnameLEN(name)-1,
               (PadnameUTF8(name)) ? SVf_UTF8|SVs_TEMP : SVs_TEMP
              );

Which should be the same as:

    sv_2mortal(newSVpvn_utf8(
                PadnamePV(name)+1,PadnameLEN(name)-1, PadnameUTF8(name)
              ));

However for some reason the compiler does something different. An
interesting subtlety is that PadnameUTF8() always returns 1. So you
would think that it could be replaced by

    newSVpvn_flags( PadnamePV(name)+1,PadnameLEN(name)-1, SVf_UTF8|SVs_TEMP);

but when I compile that it seems to randomly produce the right result or
not. I tried various formulations of parenthesizing the arguments,
reducing the ternary to its natural result (above) etc, and the compiler
seemed to produce random results. The only formulation that reliably
produced the correct results is the original one.

Author: demerphq

op.c

@@ -10962,9 +10962,12 @@ S_already_defined(pTHX_ CV *const cv, OP * const block, OP * const o,
         const line_t oldline = CopLINE(PL_curcop);
         SV *namesv = o
             ? cSVOPo->op_sv
-            : newSVpvn_flags( PadnamePV(name)+1,PadnameLEN(name)-1,
-               (PadnameUTF8(name)) ? SVf_UTF8|SVs_TEMP : SVs_TEMP
-              );
+            /* warning, DO NOT change this to use newSVpvn_flags()
+               as it will tickle a compiler bug - see patch that
+               introduced this comment */
+            : sv_2mortal(newSVpvn_utf8(
+                PadnamePV(name)+1,PadnameLEN(name)-1, PadnameUTF8(name)
+              ));
         if (PL_parser && PL_parser->copline != NOLINE)
             /* This ensures that warnings are reported at the first
                line of a redefinition, not the last.  */