op.c: Skip allocating entersub targ for Perl sub

The target of entersub ops is only used by XSUBs to return things.
Pure-Perl subs don’t use the target.  (And if a pure-Perl sub is
later replaced with an XS one, dXSTARG already has a workaround.)
op.c: Use ASSUME in multideref code

Under non-debugging builds, this gives the compiler hints about what
optimisations it can take.  With clang, I see a slight reduction in
the size of op.o:

$ ls -l op.o
-rw-r--r--  1 sprout  staff  192480 Dec 20 18:00 op.o
$ ls -l op.o
-rw-r--r--  1 sprout  staff  192432 Dec 20 18:01 op.o
perl5220delta: Function::Parameters is fixed

And it was fixed a month ago.  I missed it.