a 100Mb link is mapped to a trivial link, a linkdelay has to be
inserted since the actual loopback bandwidth is much higher. The
problem is that unlike other delays where requires_delay() will return
true, this decision has to wait until after assign does the mapping
and we find out that a link was mapped to a trivial link. When this
happens, I insert a linkdelay "on the fly" when processing the link
statements from assign.