segment to survive e.g. several nested vecnodes, which cause the base register
to change from BP to something else. In the cases where the ss: prefix is not
needed, it is removed by make_simple_ref.
+ remove the ss: prefix in the several cases where make_simple_ref isn't called
(namely spilling and tcg8086.a_call_reg_far)
git-svn-id: trunk@27714 -