Enable OPpTARGET_MY optimisation for cmp/<=>

We can only do it for <=> under ‘use integer’.

The non-integer <=> will push undef on to the stack.  Enabling
the optimisation for it would cause \($lexical = $x <=> "nan") to
leave $lexical with its previous value and return a reference to
&PL_sv_undef.
op_private: Update note about targlex and trans
lex_assign.t: Correct (s)cho(m)p comments

The were backwards.  ‘s’ means a single item.
lex_assign.t: Actually test chomp