op.c: Skip priv flags assert if ppaddr changes

because an XS module is probably installing its own ppaddr, in which
case it knows more about the private flags than we do.
Handle aelemfast_lex in S_aassign_common_vars

This was a missed opportunity for optimisation.  Prior to be9de18 it
was optimised, because S_aassign_common_vars was called earlier in
the compilation phase, before the aelemfast optimisation.
aelemfast_lex in aassign_common_vars_aliases_only

S_aassign_common_vars_aliases_only needs to handle aelemfast_lex,
otherwise we miss some rare cases where we have aliased scalars and
elements on both sides of the assignment.