pp_negate: Support magic big ints as strings

pp_negate: Support magic big ints as strings

-$1 was treating $1 as a float even if the string consisted of an
integer, due to incorrect flag checks.  It was doing the same things
with tied variables returning str+int dualvars.

Simply checking whether the privates flags consist solely of SVp_IOK
(which works for tie variables returning pure integers--so I wasn’t
entirely wrong in adding that logic a few commits ago), isn’t suffi-
cient. For gmagical variables that have already had get-magic called
on them, the private flags are equivalent to public flags for other
variables.