Pull the predecessor map and control dependence graph up out of phi
elimination, since they might be useful eslewhere.
Add a simple cdgOptimsie() pass which removes edges which the CDG can
show are impossible.
Convert realias to use the control dependence graph rather than the
control domination map.  The CDM is now completely redundant, so stop
calculating it and remove it from teh build.
Make the predecessor map return an empty set when asked for the
predecessors of the root state, rather than just crashing.
Merge github.com:sos22/SLI
Fiarly major reworking of the realias pass.  It now does a much better
job of eliminating loads.  The vast majority of atomic machines can
now have every load eliminated.