Remove OA_DANGEROUS from non-integer postdec/inc

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.

While pp_postinc may return a lexical scalar, that only happens when
the OPpTARGET_MY optimisation is happening; and aassign_common_vars in
op.c checks specifically for that.

Otherwise, it only returns a mortal or target, so it is not
OA_DANGEROUS.