pp.c:pp_split: Remove redundant !s

SvPV always returns something (or croaks), so s cannot be null here.
Clarify two panic perldiag entries
perlref: Document lvalue slice refs

Something I missed.
Optimise @lexarray = split...

‘@pkgary = split //, $foo’ gets optimised such that the split writes
directly to the array and the assignment doesn’t have to happen.

This commit makes it work also with lexical arrays.  It only works for
arrays declared previously; ‘my @a = split’ doesn’t get optimised,
just as ‘local @a = split’ doesn’t.

The pad offset is stored in the op_targ field of the pushre op, just
as the GV is stored in its op_pmreplrootu field.