Remove OA_DANGEROUS from match

OA_DANGEROUS indicates that temporary copies may need to be made in
list assignment, to handle things like:

($a, $b) = ($b, $a);

In other words, an op type is flagged with OA_DANGEROUS if its return
values could occur elsewhere on the stack.

pp_match pushes new mortals on to the stack in list context (or a
read-only boolean; read-only values don’t matter), so they can’t
occur elsewhere.  Hence it is not OA_DANGEROUS.
Remove OA_DANGEROUS from subst(cont)

OA_DANGEROUS indicates that temporary copies may need to be made in
list assignment, to handle things like:

($a, $b) = ($b, $a);

In other words, an op type is flagged with OA_DANGEROUS if its return
values could occur elsewhere on the stack.

pp_subst and pp_substcont only push new mortal scalars or read-only
values on to the stack.  Hence they are not OA_DANGEROUS.