Interpret do CORE() as do-file

a96df64385 inadvertently changed it.  do-file is how it has alwas been
interpreted, at least as far back as 5.000, as far as I can tell.
pp_sys.c: More null check removal

Thanks again to Daniel Dragan for pointing out candidates in
<rt-4.0.18-29603-1387872667-551.120842-15-0@perl.org> (ticket #120842).
pp_sys.c remove null checks and locality

pp_enterwrite, EXTEND contains a funccall, dont save gv around it
pp_ioctl, move optype to first place used to reduce liveness