Preserve undef identity in const ops

This fixes the one remaining issue in ticket #105906.

Under ithreads, the constants are transferred to the pad at compile
time.  Since the pads are AVs, and since AVs used &PL_sv_undef to rep-
resent unused slots, &PL_sv_undef could not be stored in the pad, and
so was copied.

That meant the same constant would produce a different scalar at each
call site:

$  ./perl -Ilib -le 'BEGIN { $::{z} = \undef }  for(z,z) { print \$_ }'
SCALAR(0x7fecb38062e0)
SCALAR(0x7fecb3806100)

Commit ce0d59f changed the way AVs work, so now we *can* store
&PL_sv_undef in an AV meaningfully, and there is no longer any need
for this copying hack.