[sgen] New xrefs graph optimization for nodes with incoming order 1.
[sgen] Make merging in old xrefs algorithm much more efficient.

Instead of merging the two lists by keeping them sorted, use the flag on
the SCCs the indicate whether an SCC has already been merged in, and
just append it if not.  This makes it as fast as the new algorithm on one
test case where it was pathologically slow, while not exhibiting another
pathology that the new algorithm has.
[sgen] Fix bridge time logging.  We inflated durations by a factor of 10.