pp_return: simplify arg handling code

pp_return: simplify arg handling code

pp_return() only needs to do the extra arg handling associated
with the args not being at the base of the stack frame. For example

    for (1,2) { return 3,4 }

has to cope with 1,2,3,4 being on the stack.
Apart from handling junk, everything else  - in particular pushing
&PL_sv_undef in scalar context if there are no return args - is already
done by Perl_pp_leavesub, which pp_return tail calls.

So reduce what pp_return does to the bare minimum. This makes one less
conditional branch in a few cases.