add OPpDEREFed flag to avoid double mg_get()

add OPpDEREFed flag to avoid double mg_get()

    The previous commit made various ops such as rv2av unconditionally do
    an SvGETMAGIC(). Under some circumstances this could cause a double
    mg_get() (and hence double FETCH etc). In particular, when the
    proceeding op was something like aelem with OPpDEREF, the aelem would
    call vivify_ref(), which would call magic. So in peep(), mark
    OP_RV2[SAH]V ops with the new OPpDEREFed flag if the preceding op was
    OPpDEREF. Then use this flag to avoid a second dose of magic.

    Note that RV2GV probably needs this flag too, but there weren't any
    spare private flag bits left for that op (I think).