Optimize compiler phase assembly
Optimize compiler phase assembly

I noticed in profiles that filtering the the `before` set
for hard links generated a lot of garbage in compilers configured
with a large number of plugins.

This commit filters on the fly instead.

lrytz/scala,scala/scala,martijnhoekstra/scala,martijnhoekstra/scala,lrytz/scala,lrytz/scala,martijnhoekstra/scala,scala/scala,martijnhoekstra/scala,scala/scala,lrytz/scala,lrytz/scala,martijnhoekstra/scala,scala/scala,scala/scala,scala/scala,lrytz/scala,martijnhoekstra/scala