Remove OA_DANGEROUS from loopctl-type ops

Remove OA_DANGEROUS from loopctl-type ops

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.

These operators never return, so they shouldn’t necessitate
temp copies.

(This could probably apply to dump, too, but I don’t fully under-
stand dump.)