regcomp.c: Add some asserts()

Now that inversion lists are their own scalar types, we can verify that
the parameters to their manipulation functions are indeed inversion
lists.  This adds such assertions to the bottom level code that deals
with the bare metal of the scalars.  Functions that call these (even if
only in other asserts) didn't have asserts added to them, as they call
these anyway.
regcomp.c: Remove wrong/obsolete line

This line was made obsolete and wrong by commit
a0316a6cd4a14261beb22d95530d5763e8b6756b, which changed the format of
inversion lists.  This hasn't caused problems because the code currently
is #ifdef'd out.
regcomp.c: Fix #define

cl_init_zero should not be defined as S_cl_init, just 'cl_init'.  The S_
prefix is wrong should cl_init be changed to have a pTHX_.
Remove useless assignments

These aren't used after setting.  They came from blindly copying code to
this.
sv.c: Remove duplicate dups

These dupes of two tinterpreter variables were already done a few lines
above; also move a third, related, dupe to where the others are done.